However, most of these "ageism" or "life is over after 35" things are absurdly doom and gloom. So I'd like to offer a different perspective that may be helpful to younger folks.
Hitting a plateau in your mid-40's is not... that... bad! First and foremost, I have two children in middle of their formative years, and that's where my focus primarily lies today. Maintaining a healthy marriage with my spouse, and avoiding the mistakes of my dysfunctional boomer parents. On evenings where my son has hockey practice and my daughter has dance, and so me and my wife have to work together to get them both to their respective things, I am simply NOT going to bail and make my wife have to choose between them because something came up, or I'm having to work the same hours as the VP's and C-levels in order to fit in. I have finally achieved that mythical "work-life balance", and I enjoy it.
Secondly, even if you lack the clout to DECLARE what a company's tech stack is going to be, you can still find yourself learning new tech stacks every few years. I spent the first 15-20 years of my career in Java shops, and felt like I had no agency in the matter. That's just what I happened to stumble into as an entry-level dev out of college, and from there no one would hire me for anything else. But awhile back I was playing with .NET Core, and thought, "I'd like to work with this". So I just went and found a job doing C#. It was easy to find, and I was making slightly more money as a C# newbie than I was as a Java veteran. This past year I decided that I wanted more exposure to data science and machine learning. So now I'm in a Python shop for the first time in my career, and I'm charged with giving direction and mentorship to younger developers who quite frankly are much more knowledgeable about Python. All of these things are exciting, and keep me fresh and on my toes and engaged with the field.
In a lot of discussion threads, where people bemoan that they've reached their 40's and "can't find a job anymore", I have to assume the issue is that they keep looking for substantial promotions or raises over their previous jobs, and are simply in denial about having reached a plateau (or if they haven't been willing to continuously learn new tech, may be in decline). Yes, most companies are NOT being realistic when they talk about having "parallel career tracks for managers and IC's". Rarely does the title "Principal Engineer" REALLY put you on-par with a Director or VP. It is what it is. Management will always be what people "advance" into, and individual contributor roles will always be for those "who did not advance". At some point in your career, you will likely have to make your peace with that, because there simply aren't enough TRUE "Principal" jobs (as described in the original post) for more than 1 out of 10 of us to reach.
But I really don't think that "Senior++" is such a bad fate. I'm in the top 5% of U.S. household incomes, I have wonderful work-life balance, and I enjoy my work and feel like I am continuously growing. At some point, you need to take... the... win, and stop trying to convince yourself or let unrealistic social media convince you that you're a loser.