All sorts of discrimination exists still because "perfectly rational" arguments like yours ignore underlying reasons.
For example: it's the same justification racists used for intelligence tests for voting not too long ago.
But trying to address perceived ageism doesn't solve the problem. Being aware of contributing factors and correcting for them on an individual level does.
This is a loaded and contentious statement ("assuming...victimhood at the first sign").
And trying to address ageism is trying to solve the problem; the problem is ageism.
In your particular example, ageism is manifest because older workers tend to have families and financial obligations (both of which, by the way, contribute to society). That's not "perceived" ageism. It's simply ageism.
If you choose not to work as much as someone else or made choices that require higher compensation, it is not an "-ism" for your employer to choose another candidate on that basis alone. We're going down a dark road claiming victimhood without evidence as quickly as people are.
Why should dying your hair and wearing skinny jeans affect your hire rate?
"-isms" don't work because they are rational individually, although they might work because they are stereotypically true. Or are you saying that the hiring manager should work on this individually... which would be actually just following the law.
There isn't evidence of this, but even if you believed that were the case, you combat it the same way you would any other weak point of your value proposition. You address it head on in a tactful way that resolves those concerns without having to be asked explicitly.
That could include interest and experience in an emerging tool or architecture (combats stereotype that skillset is outdated), casual commentary on the latest Game of Thrones or Silicon Valley season (combats stereotype of bad cultural fit), or a true passion in solving the problems the company sets out to solve (combats stereotype of clock-in clock-out worker). It could also include adjusting your salary expectations more in-line with what your competition is expecting (if necessary).
The possibility that ageism is not the only cause cannot be simply ignored.