Would you spend a week learning a new tech stack for an upcoming interview, with the knowledge that they might not hire you? Only to restart the learning process again next week when you interview at another shop?
The big problem is putting all your eggs in one basket, which is bad if that one technology goes out of demand (as the grandparent painfully experienced). It's not something you do for hiring in a week before interview, it's something you do all the time in the years before as a part of your natural interest and "sharpening the saw" - if you spend the last 5 years working with technology A, a person who is "capable of picking up new technologies and languages with ease" would have picked up technologies B, C, D and E during that time anyway, and can go out to employers who want B truthfully claiming that they learned B back in 2015.
For example, learning a new programming language every year for fun is a decent exercise that makes you better with your "current work tool" as well, keeps your tech stack perpetually up to speed, and isn't that hard - the second language takes significant time, the 15th (for 15+ years of professional experience) brings lots of things you've already seen, so you can do something useful on the first day and a reasonable project in the first week.