I tend to agree with you that ageism is over blown but that statement isn't a "simple" statement because ageism is complex.

There are institutional issues, where a single job can take 5 years to learn do do efficiently but company policy is to give raises every year to people who meet expectations (called a "salary ratchet"). In such a situation, if someone has been doing the same job for 15 years they probably have 10 years of raises that were "seniority" based rather than "skill" based.

The discrepancy between what it costs to get the 'skill' to do a job versus keeping someone on who has a lot of history can easily reach a breaking point. Is that ageism? Or is it a failed compensation policy? Would it be better if a company said "The max we are going to pay for this position is $X" and once you get to $X you won't be getting any more raises in this job."? That might work for some jobs, it sounds a bit like forced attrition in others.

There is also skills movement, which is to say that an engineer that has developed skills and understanding in a particular "stack" may find that when that stack is no longer current, their skill level on the new "stack" is functionally identical to a new hire, with a bonus of additional wisdom. But is that pay the same as their current pay?

People change jobs to get an increase in pay, which works until it doesn't. People rarely accept a reduction in pay when they transfer within a company. It's hard to internalize how you are "worth" $X in one role, but only worth $0.8X in a different role. So that transaction has the person leaving and someone getting hired who currently only makes $0.75X and they get a bump up to $0.8X and they are happy to come on board.

The "new" thing in HR would appear to be that salary ratchets are dead and variable pay on a fixed base is the thing. This is a fairly common situation at big tech companies these days, base pay plus some variable modifier, either additional pay bonuses or stock 'units' that convert to a variable number of shares based on contribution.

My guess is that today's new engineers will find that 10 to 15 years from now their pay is no longer growing and their bonuses and RSUs never seem to quite be all that much better than they were the previous year. Is that a good thing? Bad thing? I don't have an opinion one way or the other, but as a career, engineering is becoming a lot more 'standardized' than it was.

The way I see this expressing itself is that older engineers who are up front about being ok with more median salaries tend to get hired. Ones who insist on their 'top of the tech ladder' salary from the previous place where they worked for a decade or more, have a much harder time of it.

I know from being both an engineer, a manager, and an executive that they equations usually have nothing to do with age and more to do with work product per unit $ applied. It is easier to hire someone below their optimum value and give them a pay raise than it is to hire someone at a premium and ask them to take a pay cut to bring them in line. I also know that Google tried to do this with their 'slotting' system and it had a lot of issues (mostly negative), and that the stack ranking type systems that Microsoft used to have were similarly designed to prune toward the optimum value. Bottom line is that there isn't a good system that I know of for having this kind of salary discussion that works.