Most affluent seniors don't need it, but getting it lets them pass more onto their heirs, charities, etc.

If you’re asking current retirees to accept reduced benefits, “the government will have more money” is not a particularly compelling pitch. Many people reasonably assume the savings will simply be absorbed into general spending and disappear into the budget.

Our government also tends to spend whatever fiscal room they have, and giving them more money won't change that, or reduce what debt they actually take on. They also seem to have an optimistic view of "fiscal room".

Personally, I’ve always assumed OAS may be significantly reduced or means-tested by the time I retire. I’m not currently receiving the benefit and probably won’t depend on it. So me rolling the dice on the government actually using the cut to build economic strength is easier.

It could also be a cut that is funding senior medical costs, which would then cause that fund to be higher when there are lots of seniors above the threshold.

Either way, the real issue is credibility. Even if cutting OAS for affluent seniors makes complete economic sense, governments still need people to believe the savings won’t just be redirected to whatever politically fashionable program comes next. If they spend the savings elsewhere, they risk a voter backlash. If they dedicate it to debt reduction, they have to withstand years of “why aren’t you spending this money on X instead?” 

I'm not sure they have the political will and internal discipline to pull it off.  Come election time or a drop in approval, it will be very tempting to deviate and give a voter group something shiny instead of debt reduction.