Actually maintenance IS a kind of good idea but not as it is now.
A mean to make people play something else than max tier is a very good gamedesign decision, and something needs to be there to incite money spending, but NOT with the numbers there is right now ; the whole thing should be more of something like:
everything should be more expensive in the low tier (but cheaper than the highest tiers)
better money rewards across every tier
maintenance everywhere scaled on the total tiers in your fleet, with high maintenance still in elite tier, and starting at tier 3
no tech walled beyond tier 3 (mortar turrets, ...) but still at t3 to not have the whole experience in recruit league (to incite to progress), basically moving everything one tier up
no ship walled beyond tier 3, basically moving everything one tier up
This way, you'll make people play in high tiers for fame and freudian feelings of power and sexual compensation, but they'll have to go back in lower tiers, with everyone, to make money and pay maintenance after a strike of lost games in Elite. Recruit and Veteran leagues will always be populated by players, including good ones, but without ungodly raw number advantages against noobs - and they'll try to play at tier 3 anyway except when broke or farming.
Xp conversion becomes really interesting by training on the less advanced versions of a ship (lore and money-making friendly) without being an obligation.
Elite status allows to remain in elite league far longer even when losing and is still an interesting thing to buy when fan of the game without being an obligation. A lot of people will buy short time periods of premium to play a week end in a row for example.
The gameplay differences due to varying ship speed (lower tiers being quicker) will even still be interesting because the gap will be less of a chore in veteran league.
Oh and of course, something nice with the fracking hero ship ! (More credits ? Lesser maintenance ? The other way around, something at the cost of MORE maintenance, being custom famous hero ships ?) This way there's a reason to buy them that is NOT pay to win, but pay to gain more time, or pay if you have little playing time because of your job/family (but then more money...).