In my opinion, I feel that granting mechanics the ability to upgrade cars gives more role-play for everybody. You look at NoPixel, and the FamilyRP, they have certain people who upgrade cars and it's always granting more role-play. In a city like K33N, more role-play is what you want EVEN IF you have to spend a couple more bucks for a simple upgrade.
Yes, I agree it is more of a hassle to try and upgrade your car, but overall, it's definitely way better for the city in terms of role-playing. Not to mention, mechanics didn't even get paid for fixing your car up. Forgive me if I'm mistaken, but I believe now that mechanics can upgrade cars, that is their ONLY way of making some kind of money around the city in terms of their job. Fixing the economy seems like the #1 priority for K33N in the moment, and having to go through mechanics for an upgrade is just another step towards making the economy flow a little more smoother.