Why do you need all 20 cars fully maxed at the same time? All players got refunded the same amount of in-game currency which is the exact amount that they had to spend on the old performance mods. All players are in the same situation whether it's a credit car or not. Credit imports never ever included any mods in the package, but I made an exception for the HSW mod due to it's extremely high cost (sometimes up to $1M per mod). I also don't think it's reasonable to say that you lost 1000s of euros because of the pricing changes. The mod pricing changes do not obliterate the car value to 0 credits, many credits are bought during sales, credit prices change (Niobe used to be 25k, not sure how much you spent personally), the refund money mitigates part of the new upgrade costs.
If you received refunds from 20 fully maxed cars, then there should be enough refund money to fully max at least some number of those cars, it is up to you how you prioritize them. I noticed that some cars are not that expensive and cost much less than 100k to max (which is the estimate refund per fully maxed car), so that also reduces your "losses" and you shouldn't even be listing those cars in your complaint at all.
What I would suggest is to prioritize maxing the cars that you actually use frequently instead of just maxing them all at once just because (practical vs emotional). You may also prioritize the EMS (Engine) mods and Turbo and leave the Brakes, Suspension or Transmission for another time which would allow you to save money, but have near similar performance as before. Mods are also incremental, so lets say you can also consider having LVL 4 mods on some cars instead of LVL5 on all cars. You can always upgrade them later once you have enough in-game currency. If maxed out cars really matter to you that much, consider selling an in-game asset or two to get the upgrades.
Overall, you still have the same cars that you bought, they still have the same stats and mods available, but the pricing of those mods has changed due to a balancing change which is covered by the credit purchase agreement under development and server rule changes.