As soon as these are released to market on any World, shouldn't anyone with enough credits be buying one or more? I mean, in real life, the iPhone is flying off the shelves as soon as it's released.
The correct answer to your question is ... it depends on how many credits are involved and how secure the supply chain for imports is.
Without enough credits, importers are not going to sell to you. Full stop.
The "supply chain issues" basically amounts to a matter of Market Size (how many OTHER people are demanding the same product) creating enough of a demand signal for a supplier (or several) to emerge to meet that demand (and scoop up the market for those credits). As others have alluded to in their answers above, without a network infrastructure

your iPhone loses a LOT of its capabilities which rely upon (external to the device itself) network services. If you're getting "zero bars" of signal to your iPhone, MOST of what you would want to have it for (not all, but most) will basically "stop working" when you can't access The Network™.
Would anyone be using old or substandard equipment if they can afford otherwise?
If they have infinite credits and unlimited contacts/friends/patrons who can obtain better for them ... then NO.
Does that describe EVERYONE in existence?
Oh
hell NO.
Will people be using "what they can obtain and afford to use" ... probably.

Note that "afford to use" is doing a MASSIVE amount of heavy lifting in that assertion, because that "afford to use" point includes BOTH the "got enough credits" as well as "supply chain issues" points that I just raised above.
The Tech Level of a World has to do with Production, right? Not Import power.
Almost ... but not quite.
You're oversimplifying things a bit too much, I reckon.
The way I see it is that Population: 6- (Non-industrial) worlds have "immature" economies which must import a lot of their finished goods.
LBB3.81, p16:
Non-industrial worlds are forced to import much of their finished goods.
What this means is that Population: 6- world economies are primarily oriented around resource extraction and are often "economically dependent" upon other star systems (for finished goods, for starters). In broad brushstroke terms, this means that Non-industrial worlds tend to function a lot like "colonies" that produce raw materials and then trade those raw materials for finished goods.
Population: 7+ worlds are the ones with economies "mature" enough to have diversified into the production of finished goods.
Industrial worlds have LARGE production bases for the manufacturing of finished goods ... but they need a lot of resources to "power" that production, which are (of course) obtained "more cheaply" from Non-industrial worlds, creating a natural flow of supply and demand in both directions.
The Tech Level of a World has to do with Production, right? Not Import power.
Like I said above ... almost, but not quite.
Tech Level measures the level of technological sophistication you can EXPECT TO FIND on a world, meaning that products of that tech level are "routinely obtainable" (unless restricted by Law Level, of course).
Whether or not a world is the PRODUCER of products of that tech level, using the world economy to Make Stuff™ ... comes down to a question of whether the item in question is "raw materials" or some kind of "finished goods" to be bought and sold. Most manufactured items (particularly high tech ones) will tend to qualify as "finished goods" and will thus tend to require a Population: 7+ world to manufacture and supply them.
This point is actually made explicitly in the Travellerwiki in the
Missile and the
Anti-shipping Missile articles in the History & Background (Dossier) section of each article. I'm presuming that such notation is
not an accident or otherwise in error.
So to answer your question more precisely:
- Population: 7+ worlds produce finished goods up to their tech level, subject to law level restrictions.
- Population: 6- worlds import finished goods up to their tech level, subject to law level restrictions.
However, that's a BROAD generalization. It's perfectly possible (in theory) to obtain incredibly high tech imported "stuff" on a low tech world ... but the price you would need to pay for it ...

... forget about the arms and legs, it'll cost you
your entire torso.


If you've got the credits to pay for that kind of
usury highway robbery arbitrage ... there's probably going to be a "supplier" willing to import whatever you want (at an appropriately exorbitant price, of course).

As soon as some new Tech is even advertised, wouldn't the big MegaCorporations be selling it?
Depends.
If it's military tech ... probably not. Law levels might get involved.
That's why you can't find plutonium in every corner drugstore.
If it's "civilian tech" ... possibly ... but it depends on how "willing" world markets are to import (and embrace) the new product ... and what sort of Entrenched Interests™ they might have to go up against in those markets they want to export to. For a current day/real world example of the kind of "resistance to technological advancement" that I'm talking about, look no further than the Battery Electric Vehicle revolution in ground transportation and aviation happening RIGHT NOW all across our globe. Look at the Solar+Wind+Batteries revolution in power grid technology happening right now ... and all of the pushback coming from Entrenched Interests™ that don't want to be made obsolete (and kicked out of the market) just yet.
To put it mildly ...
it's not that simple ... or at least, not as simple and your question blithely postulates the matter as being.
The term
Luddite exists for a reason ... and that reason isn't necessarily confined to past history.
When the livelihoods and fortunes of Entrenched Interests™ get threatened by innovation and disruption, those Entrenched Interests™ tend to band together to FIGHT BACK against the innovation and disruption that threatens them. After all, no one wants their "gravy train" that they've been riding on (comfortably) to be "derailed" and stop working.
In short, this is not a One Size Fits All type of question and answer.
There are a LOT of details to consider ... including the age old
"Not Invented Here" xenophobia that even corporations (mega or otherwise) can fall prey to (partly as a means to "defend their turf" and sustain themselves as an Entrenched Interest™).
And wouldn't the Tech Level limit on Equipment in Character Generation not really make sense then?
Distribution of goods is not ... uniform ... across all worlds within a single polity (pocket empire or otherwise).
Character Generation (LBB1) was designed to be "origin agnostic enough" to be plausible in as wide a range of campaign setting circumstances as possible, rather than being tailored for a specific OTU location, era or setting.
A better answer would be that if a Player can concoct a rationale that passes muster(ing out) with a Referee, then some "bending of the rules" to enable a specific outcome (I got better tech level equipment as a result of character generation) becomes possible ... but it's going to take some EFFORT on the part of the Player to concoct the details of that specific part of the character's backstory and history which the Referee then has to approve of.
In other words, it CAN be done ... but there are "no guarantees" by default that it will be permitted.
