That's a nice ideal but it doesn't work very well in practice. It takes an enormous investment to become an expert in any given technology stack, easily on the scale of attaining a college degree. When a restaurant needs a baker, or an executive chef, or a pastry chef they hire specifically, not generically. So does the tech industry. If a company has a rails stack and they need someone up and running quickly it does no good to say "I can do it, just give me a few weeks to get up to speed and a few months to get really good".
Plenty of companies go for talent over niche. When you expect the average person to last 5+ years it's dangerous to only higher people that know exactly the stack you work with every day, because it narrows everyone focus and they ignore edge cases where it's really better to use some other tool.
Sure, but there are limits. If you have a company that is heavily focused on web dev, you are naturally going to want people who have web dev experience. Maybe you won't mind too much if someone with a lot of talent has a history with Python or PHP and your current systems are based on Rails, but you will probably be a lot more cautious if someone applies with a background in programming IBM/360 systems and knows nothing about html or the web?
I do think it's important to hire on talent, but I think the other extreme of a super generic "computer-based problem solver" is very unrealistic. Not every company can afford the delay of bootstrapping someone into having expertise with the technologies the company uses.
