I am a strong proponent of ditching old hardware when I find myself no longer regularly benefiting from their availability.
Also, I am a strong adherent to minimizing system administration load.
Today, I have one desktop computer (Mac mini 2018) and one notebook (MacBook Air 2019). There are no other Macs in the house. There is one cheapo Wintel PC in the house.
That's enough for me.
If you want to keep your old gear around, that's entirely your prerogative but I have better things to do with my time.
In the past twelve months I have ditched: Mac mini 2010, MacBook Air 2013, MacBook 2017. I am glad they are all out of my life. I don't go looking around for a reason to keep old computers. If the usage case isn't obvious (home theater PC for the Mac mini 2010), then it goes bye-bye.