Joke aside, I don't know how much of this problem is more related to older devs insisting to have the same dev employment they always had than acutally being old.
Go into management.
Become self-employed.
Start your own shop, service or product based.
Teach younger people what you know for money.
There are many jobs besides just being a "senior developer".
I've also seen many non-tech occupations becoming more technical.
Learn something unrelated on the side and try to apply your dev skills to it.
Being a dev is cool, but being a dev-lawyer, dev-md, dev-techer etc. Is probably gonna let you have a good job later.
I believe "plan b" is a head first compound noun, and should pluralize like "hangers on", "mothers in law", or "attorneys general".
But I could be wrong. The plural form doesn't get much usage.