The Apple Watch will continue to work just like the day it was purchased for 6-10 years without problem. There are forum users still using Series 2 watches with iOS 26.
How will they "regret" it? New hardware comes out all the time and those who bought previous versions won't have what new versions have. Watch OS is very hardware focused.
If you're trying to say - buy the latest chip design for longest OS support, then ok, but it's not like the watch will stop working - it'll work just fine for a very long time.
I'd also add, you can sometimes get amazing deals from Best Buy $100 off - and get a current generation Apple Watch for a really good deal.