The flaw is simple: you'll end up with only having as criminal defence attorneys those people who didn't succeed at being civil case attorneys, as everyone will prefer to earn more money with mostly the same training. So you won't have any outstanding criminal defence attorneys, which is equivalent to undermining the rights of those of need to defend themselves in criminal cases.

If you want to go this way, you have to go for the full-fledged solution: either the defendant choses their attorney (at a capped price) or an attorney shall be selected at random among all attorneys registered with the bar and must take charge of the case, the only exception being if they have a valid conflict of interest in that case.