Speaking for myself, the motivation for such thoughts mostly stems from treating wealth as a reward for work. Which is not crazy, it's similar to the labor theory of value. Or if you prefer, incentivizing work means more work will get done, and work always needs to get done. There are valid reasons from a number of perspectives to valorize and reward work.

And the elderly largely don't work, because they can't anymore.

Shifting services means the elderly will get poorer. Which given the high wealth level they have in developed countries relative to the young is not a catastrophe. However it would be a huge adjustment to expectations downward, since the elderly would have the rug pulled out from under them. Whereas the young haven't yet had a chance to adjust their expectations up since they haven't had money yet. Hence the political settlement.

For a prosaic example, in many US states funding for public universities has been cut to pay for public sector pensions in the past thirty years. This has shifted educational costs to the young in exchange for maintaining the benefits of retirees. This is an example of shifting services from young to old.