If they brought back the credits they should be worthwhile amounts.
Demo cars unlocked.
Lets say you'd need about 1000 points to get the TBO class, 4000 points to get the LX's... 50,000 credits to get the BF1.
That would take most people, even in one lap races against 'Pro' AI, a few months on constant play to get, thus by the time they get to the 'fast' cars they have at least got a vague idea what they're doing.
If you want it to remain 'easy' to get the cars, then don't bother, and just have them all open as now.