S4, My experience is that most players I've run for will keep going until they get that 4+ term guy they want, UNLESS you use the alternate survival rule.
Short term tends to put YOUNGER characters in play.
I've experienced the opposite, although I see where you're coming from.
I think the difference is that, with the people I've played with, they will rarely just try to kill off a character just because they didn't like him.
If I saw that happening as a GM, I'd impose a little house rule. Something like: "If you fail Survival, you take a negative DM to a random stat equal to the number of character's you've tried. Character #2, you'll roll 2D for five stats and 2D-1 for the sixth. Character #3, you'll roll 2D for four stats and 2D-1 for fifth and sixth random stats, and so on."
The thinking here is: If the players need a reason to respect the Survival Rule (and they don't already), then give it to them.
My experience, though, has been that this type of thing isn't necessary. I've never implemented a rule like that. But, then again, the people I play with don't try to kill of characters just because they don't like them, or rolled low, or didn't get Pilot skill, or whatever.
And, I should note...
Even if a player does play that way--to where he will roll stats, not like what he gets, and then join the scouts, reenlisting until he dies--so what?
He will finally get some stats he likes--and make a "connection" with that character. And, when that happens, he'll fear the Survival Rule and have it color his choices just as it should.
A GM suggestion...
If generating Classic Traveller characters by the book, I suggest using the rules as written (no house rules, and no optional Survival Rule), but do this with the group as a whole.
If you've got a player who will tend to kill character after character until he gets what he wants, there will be a certain amount of peer pressure to not act that way. It's much easier to keep rolling up characters when it's just the GM and one player.
Plus, character generation takes longer when doing it as a group. Don't work with one person individually. Work in terms. Do Term 1 with the first player, then the second player, then the third, etc. Then, go through and do Term 2 with all of them.
This allows the player time to think about what he's doing as he watches you work with the player to his right.
One last thing...
Even if a player kills off a couple, I've seen a player start a Term, trying to get his character killed, and then...some type of...magic happens. The character won't die. And, the player begins to make a "connection" with the character because of this.
All of a sudden, the player is trying to keep him alive--and you hear something like, "Oh, I'm not going to re-enlist. I like this guy. He turned out OK after all."