As a non-Boomer (Gen-X), I have to defend Boomers a little bit on this. Although a few sold their SDGs to CMGs, many (if not most) of them just lost their contracts, were replaced and got little or nothing in return. Why would a CMG pay to buy an SDG, when they can simply undercut them and replace them without a fight? Once they've crippled an SDG, they can offer to buy out the non-competes and their doctors at a fraction of a price with little recourse.
SDGs by and large never came up with an effective way to prevent having their contracts terminated and quickly replaced. The grand plan of using non-competes didn't work and never had a chance of working, for SDGs that left themselves vulnerable to a sudden 100% drop of revenue due to contract loss.
SDGs were paper tigers all along. It's a miracle any of them lasted as long as they did, run by doctors with little to no business skill and even less corporate-killer instinct. We are where pharmacists-owners were 50 years ago; being replaced by corporate pharmacy chains. Just as mom-n-pop grocery stores aren't coming back to slay Walmart, I seriously doubt doc-owned SDGs are coming back to slay CMGs.