Outside of work on my own time I play with things I think might be useful or worth using a year or two from now, if they are fantastic, if not I learnt something and had fun.

At work it's PHP (I inherited that project sadly), C# and Java, On my radar in the next year are Kotlin and F# (Kotlin particularly since we have java applications in production on industrial handhelds and the codebase previous dev left on those is...worrying).

As for people screwing their employers by picking unproven new shiny, I think in the main part that's because of mismatched goals, Employer wants reliable software but doesn't understand what dev is doing and Employee wants a resume that will get them hired so skates to where the puck will be on the ooh shiny.

I've been programming since I was a kid in the 80's and the one thing I've learnt (often the hard way) that no language/framework/library is a substitute for thinking things out, that and everything is a trade off in some direction.

A 5mm square A4 pad and a pack of decent coloured pens has saved me thousands of hours over the years.