As I recall, in all of those, I saw one character survive more than four terms, either due to failed survival rolls or advancement rolls that dropped them out (more the former, than the latter, of course). One. 3%. Close enough to 'nearly impossible' for my books, thanks.
This 'failed Survival roll' argument is totally bogus. If you fail a survival roll, you miss out on benefits
for that term only. You can still muster out with benefits from the other accumulated terms and promotions from that career.
You can still choose to carry on with another career after. Even if you fail the Qualifications roll you can still get drafted or become a Drifter. I've rarely encountered
any player forced to quit generating a character after four terms only.
Given a standard character will have about six terms of experience
on average (without Referee stipulated limits), it's also reasonable to state that a typical character will accumulate about one benefit per term
on average. Some characters will have Mishaps, and forfeit benefits rolls, but this is averaged out comfortably by the extra benefit rolls you can accumulate through advancement too. That means they will typically have about six benefits to roll come mustering out. They can spend up to three rolls on cash benefits (which in my experience they usually take), giving them three rolls on average, on the Other Benefits table.
Now, the Other Benefits table vary for differing careers. If you choose Marine or Army characters, you won't get any opportunity for ship shares. For other characters, like the Navy, Citizens, Nobles, Rogues, Scholars and Scouts, you get multiple entrees. The other Careers get at least one entree, out of a table range of 1-6 (or 7 with a +1 benefit). Given the propensity for characters to mix careers, I'd say that
every character has a decent chance of rolling ship shares as benefits. Usually, these benefits are for 2 or sometimes even 5 ship shares at once in a single entree. One (or even more) chance out of six, for any benefit roll to have ship shares, is a pretty decent chance to get some.
That means, for a crew of five PCs, with an average of
fifteen Benefit rolls between them, the chances of getting as low as five shares between them is
statistically unusual. I'd say, for a crew this size, accumulating ten ship shares between them is about
average. This is backed up by my experience and is hardly 'impossible' at all.
Of course, if the crew have all specialised in taking army or marine characters exclusively, then this is going to impinge upon their chances somewhat. But conversely, if they choose to take Navy, Noble and Scout careers then the reverse is true also.
Either way, having an
average chance of getting ten ship shares, valued in 10% increments is likely to disprupt many games. The chances of 2.5% is obviously less, but frankly the rules of the game are what they are.
If you want to change them to what you want then cool, but y'know dismissing the game as 'Traveller: The Accounting' is really just dismissing the process in which character advancement in the game works. The default game is
about playing a group under financial pressure to pay off their mortgage at the start of the campaign - with opportunities to increase wealth, month by month, through adventuring.