On the one hand, I'm highly skeptical that it's actually true. I mean, it's not as though the dealer is the only place you can go for car servicing even now. In fact, going to the dealer is generally considered to be what suckers do. And in terms of complexity, cars are likely peaking out--as we transition to electric, they'll get mechanically simpler. Whether the electronics are serviceable (software lockout, etc.) is an interesting question, but the only way to make independent service operations viable is to mandate that manufacturers make replacement boards and diagnostic manuals available. If we don't, manufacturers will be able to software "lock out" everybody no matter what our sales channels look like.
Even assuming it's the case that we need some mandate to make it happen, there's no reason for car *sales* to go through that channel. You just need to mandate that manufacturers make parts and manuals available to third parties on reasonable terms. The reality is that the dealers don't seem to be making much money on sales anyway these days. It's all through servicing or financing. And in exchange for that, we get a miserable, sleazy buying experience that is widely recognized to be the worst in any industry or market. So it seems clear that whatever we're trying to do, we're doing it wrong.
It seems like the simplest solution would be a ban on manufacturers offering services and parts directly to the public. That way there would be a profitable market for parts and technical manuals, but the manufacturers wouldn't have an incentive to kill off the competition. The service operations would just be customers instead of competition.