We already do that. I don't see how the preclin and half a year of clin contribute any meaningful information to students.
The MD, like the DDS, should grant one the privilege of providing general medical care. Medical education is ineffective, medical school is a pointless exercise in futility and sucking-up, and the curriculum is based on a century-old recommendation paper.
The dentists are out day one post-graduation ready to practice. Their craft is not easier than ours. But their schools actually teach them how to do it, not how to suck-up and shadow people. Anyone who trumpets how medicine is so much more complicated and cannot be practiced without a residency in family medicine is delusional and hasn't thought this through.
Three years of medical school. Two years of "family medicine" residency right after. Everyone gets the MD and general practice privileges. No match required. Wanna specialize? Apply later on, just like every other job on the planet.
So you're proposing extending general medical education and requiring everyone to do 5 years before they can go into a given specialty instead of 4? Interesting.