I agree with you completely.
The OTU could have remained an exemplar, loose setting, instead of the contradictory sprawling mess it became (often due to third party authors making up stuff).
Most of the Third-Party stuff was decanonized in 1984... Like all the Judges Guild sectors. (which, really, were the best thing JG did besides the screen).
It's worth reiterating, tho', much of the OTU-ish feel is strongly encoded in the jump mechanics.
What little Foundation I've read works with Jump alright; it's a pretty close ringer for the Codominion Universe's Alderson drive (but note that Alderson Drive is a bit finicky about entry exit points; add that, and MiGE is pretty much about straight TL 16... due to the screens. Yeah, we love the MacArthur... because we can build the ol' Mac.
Traveller tech is too low for Star Trek (even in 1E, it's at or off the charts).
BSG tech is pretty firmly around 12, but has a different space drive, FTL commo, and a number of other things that don't work for Traveller.
Buck Rogers is yet another space drive and FTL commo setting. Both hyperdrives (on big ships) and stargates. Stargates in Traveller are TL 22+... (Adv 12). But even if someone wanted to, it was tied closely to TSR by 1984.
Dune, popular as it was, was a wacky tech paradigm. Not a good fit.
The Fuzzy series looks like a good fit, but it's one I've read but not committed to memory well.
Tweaking CT to do other settings is not a trivial task, unless those settings have no FTL commo, and no or slow FTL drives, no teleporters, etc.
The best results from Traveller as quasi-generic came from simply rolling up a setting, and using what's there. It may not be the OTU, but it's no worse a play experience. I've heard lots of complaints about using it for other SF settings. Including my own players. (Star Trek really isn't a good fit. Even tho the speeds in TOS are actually comparable. The tech paradigm really is quite different.)
By 79, the rules were being written to support a big ship OTU... when I came to Traveller, most of the people I knew considered Traveller to be the OTU. I wouldn't encounter a non-OTU game run by anyone else until 1994. And I ran a Trek game in '90, and (I'm almost embarrased to admit) a Tron session in 1984. And I was able to use Car Wars as a Traveller addon... (1 CW DP = 2d6 personal combat wounds.)
My Trek conversion involved a rescaling of the tech levels... Jump didn't exist, but Warp did. TL 10 was Antimatter plants. M-Drives were 1 G and rating x 10 PSL. PP Fuel was 5 years in the drive itself. Ship Phasers were meson gun equivalents. by the time I got done, it was unplayable. It was the wrong tone of mechanics. Traveller mechanics impose a strong feel, and it didn't overcome it.
The Judge Dredd conversion for Mongoose feels VERY wrong to me; even more wrong than their D20 one. Fortunately, I've still got the old GW Judge Dredd RPG.... when I want a dredd fix, I use it. Despite having both the others. What's wrong? They upped the complexity. They make age 18 several terms done. They issue skills at T4/T5 rates for Judges... but not for crooks. Only a surprise attack has much chance of even making a judge worry...
One conversion has promise; the Bab5 one. It feels incomplete.
The Slammers one lacks the rules for integrating the new tech into the (released later) vehicle design system... and it's not too outré... but it's also a setting very much not about spaceships.
When even the pros seem to botch adaptations... is it truly generic?
When it's bloody hard to adjust the tone due to setting encoding in rules, it s it really generic?
I genuinely think the claims of genericness are mere lip service until TNE's FF&S. T5 is the most generic ruleset yet, but still, it has a lot of setting encoded and enshrined in rules. And not in the fluff, either. Too much to really be generic.