Regarding slashing provider compensation, if Medicare payment rates are so low, why do providers participate even now? Need the business?

Because the government has a monopoly over senior health care spending, which is a huge chunk of the market. Well, technically a monopsony - since government is the sole buyer, rather than seller. But same thing. It gives them enormous market power to dictate terms. In the comparatively more "free" market of private insurance, where providers meaningfully can choose which panels they which to participate in and which ones they stay out of. That's especially true for hospital providers, where they actually have the dominant market power themselves - a private insurance network cannot go without including the regional hospital, which gives the hospitals market leverage to get good reimbursement rates.

Moving large numbers of patients out of the private system into Medicare will significantly reduce provider reimbursements. There are a lot of consequences to that: marginal hospitals and providers will come under some financial strain, some other providers (probably not hospitals) may exit the program, and you'll see an enormous amount of political pressure to raise Medicare rates.

Yes, medicare buy-in is a fantasy, not likely to happen. Would be quite nice for me, though.

That last statement is one of the real sticking points - most people think that being able to participate in Medicare would be "quite nice" for them. But one of the bbig reasons that Medicare is so popular is because it charges a premium that's only a tiny fraction of the cost of coverage. You get $19K worth of health care coverage for $3K. That's "quite nice" for anyone. Medicare doesn't charge an actuarily fair premium. The reason seniors are able to get that price is because they (as a group) have been pre-paying into Medicare for their entire working lives.

That's the political danger of proposals like Krugman's - or "Medicare for All" in general. Some people's perception of Medicare is "super-cheap health insurance." If people think that getting to participate in Medicare means that they'll get the chance to buy health insurance for fifteen cents on the dollar, and that's why they like it, they'll be disappointed.

If they were to charge actuarily fair premiums (as Krugman suggests), Medicare for All shouldn't be quite nice for most older folks in the ACA right now. It would be vastly more expensive, because an actuarily fair premium would undo the subsidies and community rating benefits that keep ACA old folks' premiums below those levels. It might have some advantages (wider networks), but those advantages would have considerably high costs.