There should be a continuous system of upgrades - Once you done all upgrades, you can upgrade that one to lvl. 2 e.g increase customer rating by half-again at lvl 2. Or, at least, you should be able to level people up or buy things with exp once you've bought all the upgrades.
It's kind of a waste just amassing all of that exp and not being able to use it.
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 "Learn to Fly 2"){kind=icon}