While I see what your saying, a lot of your language seems to come from the idea that tech is a progression, that we are moving "ahead" always, and that people

While I see what your saying, a lot of your language seems to come from the idea that tech is a progression, that we are moving "ahead" always, and that people get "left behind". That's mostly not true. While there definitely have been advances, tech is also very faddy and repeats itself. Trends are often reactive, techniques repetitive. Someone who has been around for long enough to see some of these waves resolve themselves is in a much better position to evaluate current and upcoming approaches. Technologists also tend to be myopic, and view the things happening in their own tiny slice of the industry as "what's happening". There is a whole universe of interesting stuff on the go that you are at best barely aware of. Personally, if I'm hiring senior technical people I'm much more interested in their continuing level of curiosity and engagement with something, rather than particular stacks. So maybe you are worried that your candidate who has been doing ASP.NET WebForms for the last decade on IIS will have a hard time getting up to speed on your use of TypeScript and React ... but if I find out that she's also being building raspberry PI infrastructure for fun and messing around with the Julia language to implement a cellular automata project I'm not all that worried about their ability to get up to speed, even though none of that stuff will get used by the team. But yeah, it doesn't take 3 weeks. It doesn't actually take 3 weeks for a new dev already well versed in your technologies, with rare exceptions. At least not to be firing on all cylinders. The thing is, a huge number of teams really, really need a good senior dev. They often don't think so, and often erroneously think they already have some... but they don't. Instead they have people who had "Senior" (or "Staff" or whatever) added to their title because they'd been there for a little while. And every piece of work the team produces suffers for it. So yeah, it's typically worth the investment. Now granted, that assumes you are good at identifying talented people. This is actually a big ask - and one of the issues surrounding the OP's questions really comes back to that. I think that a lot of software and technology development suffers fairly systematically from poor quality middle management. One of the symptoms of this is the "ageism" discussed here, but it's only one of many....