Krugman on Medicare vs Vouchercare

Krugman [NYT] makes a key point on the repugican promise to "only" replace Medicare with VoucherCare for the under 50s, it's a promise that won't be kept:

Still, the repugican cabal promises to maintain Medicare as we know it for those currently over 55. Should everyone born before 1957 feel safe? Again, no.

For one thing, repeal of Obamacare would cause older Americans to lose a number of significant benefits that the law provides, including the way it closes the “doughnut hole” in drug coverage and the way it protects early retirees.

Beyond that, the promise of unchanged benefits for Americans of a certain age just isn’t credible. Think about the political dynamics that would arise once someone born in 1956 still received full Medicare while someone born in 1959 couldn’t afford decent coverage. Do you really think that would be a stable situation? For sure, it would unleash political warfare between the cohorts — and the odds are high that older cohorts would soon find their alleged guarantees snatched away.

Or to put it another way, Romney plans to break the Medicare promises made by previous repugican presidents to people born since 1957, so what is to stop the next repugican president doing the same thing and breaking the promise to those born before 1957?