In my theoretical scenario where they get to keep your car if convicted, if they gave it back to you before trial, most people would hide it or otherwise make it unavailable for seizure. If you didn't get convicted, they should give you the vehicle back for free (you didn't do anything wrong, let the government pay for the improper charge). It's just an idea on how to try to even out the pain among citizens. Again, this only works if the trial date is very shortly after the charge.
Even geared to income is problematic as many super rich people have little declared income and huge net worth. Net worth is far to complicated to assess. Losing the vehicle you were driving eliminates all of the hassle of establishing net worth or income, or whatever other metric is based on ability to pay.