My wife recently joined a massive, multi-national firm at senior management level and was handed a......blackberry! It's brand new but is a 6 year old model that's probably been lying in a store room at company HQ for years.
Point is, it works perfectly and actually makes me miss the Blackberry OS. It's still supported by the manufacturer even although they killed the platform 2 years ago.
On a similar note, I have two very large clients here in the UK (40,000+ employees) and they're all on windows phone 8. They have absolutely no plans to change this. The operating system and it's basic apps still work.
Heck, my Lumia 800 still works
W10M may wind down but your phone will still work a decade from now