It's not like the information is unavailable. You don't have to mount an expedition to the Himalayas or anything. Just go look it up. You may well find that you still think these laws are bad and should go. In that case, you can rest assured in the knowledge that you now understand the issues better, and you're better prepared to argue your case.
Arguing that you should know why something was created before you remove it is actually the opposite of a mental shortcut. It's saying that you should do your homework before you make changes.
As other comments in the thread have made clear, the "research" has been done, and the ancient lore of how automobile sales were different from every other aspect of the economy a century ago has been rejected.
If the research has already been done and supports your desired outcome then why are you fighting it? The argument is not that these laws were enacted for a reason therefore they're good laws that should be kept. The argument is merely that you should understand why they were written before you argue against them. If you already understand that context then you're already doing what's being suggested.
Conservatism is fine as mental shortcuts go, but we shouldn't take every mental shortcut available. We don't actually have to worry about shit that supposedly happened a hundred years ago, if we take the time to look at what's going on now. Right now, there are customers who want to buy, and producers who want to sell. Relaxing dealer requirements would allow those beneficial transactions to occur.
If we're so concerned that unspecified bad shit will happen because who knows it might happen, then simply put a time limit on the relaxed requirements. That is, the law could say that dealer requirements will come back in force in five years, unless subsequent legislation extends the period. In five years, legislators and lobbyists and maybe even the public can conclude "yes the world is exactly the same now as it was in 1920 so we need exactly the same laws", or not.