If the standards were not an issue then KY would have never spent the last few years updating the 800 standards. All 800 standards have been updated one way or another in GT5. Added generic interiors, some have better interiors, wheels can now be swapped, and more. Clearly its an issue when its 80% of the game. If it was not an issue it would never have been touched. That's not including whatever they have improved from GT5 to GT6.
Actually you can NOT complete GT5 without using standards. Its impossible. So YES you are forced to use standards if you want to make progress in the game. You know how quick the campaign would be over if you only used the 200 (20%) available premium cars? Good luck with that.