The iPhone 6 Plus is a 6 year old iPhone. The smart phone tech has been advancing at a rapid rate since the original iPhone was released in 2007. What they could pack into a low power chip set in 2014 is just not enough in memory and CPU/GPU capabilities as current iPhones. As it is the 6s has "just enough" extra RAM (not storage) to support iOS 14. Not sure how much longer that is going to hold true.
I just replaced my iPhone 6s (5 year old iPhone), but my Wife is still on her 6s. If next year, the 6s drops off the Apple Watch supported list, she will be upgrading without question, as we really REALLY like our Apple Watches.
Having a 6 year old iPhone and deciding to upgrade is not being reckless with your money. You would reserve that designation for users that have to upgrade every year, or even every other year.
You have done good with that iPhone, and if you can live without an Apple Watch, then the iPhone can still serve you for a while longer. BUT start budgeting for a newer iPhone in the moderately near future (1 or 2 years), because eventually you will either want one, or your will suffer a failure that costs too much to repair. Being ready for it, can make the lost easier to take.