The only rational measure of the harshness of the cut in benefits is the percentage reduction in benefits. Krugman was saying that the increasing harshness of the cut won't match the increasing wealth of the person, which is a hardly a reason to dislike the proposal. It looks to me like he desperately went fishing for any numbers that look bad, knowing that his target audience wouldn't know enough math to correct him.

Yes, but he ends up being dishonest because Social Security income is already capped at $90,000 both for tax purposes and for the purposes of calculated benefits. Given that, it's absolutely impossible for any cuts to not be a higher percentage of income for those below $90,000 than those above it. If you change the comparison to the cuts as a percentage of contributions, it's quite different.

Even these cuts keep benefits growing at the pace of inflation, remember.

As you correctly note, one of the problems is that Social Security defenders can't decide whether Social Security is an insurance/welfare program, or some sort of forced retirement savings scheme. If it's the latter, then you have to look at contributions and it's certainly not unfair to the middle class. If it's the former, these changes clearly move it in the direction of being an insurance/welfare scheme. I agree that it should be a welfare insurance program designed so that seniors don't fall into poverty just because they lived too long. I think that the President's plan is a good first step towards getting there.

The most rational thing to do might be to make SS strictly a welfare program, with no money going to those already willing to support themselves. I think the political argument against this is that people will be more in favor of cutting benefits for the old if they don't see themselves as in line for those same benefits. Given what's happening to Medicaid, this argument seems to have something going for it.