I’m 69 and I’m still riding older bikes, including mtb’s. Here is my take, when the time comes to upgrade to a better bike, you’ll know when it’s time.
When you say you could clearly continue to ride your loving companion, whether lifeless or not, you forgot about the complaining while you ride. When you get to the point of posting about it, that time to move on is there.
My only advice is to research, try out a few production bikes to get an idea of modern, set your sights on what you want, and just do it...
... and have fun in the process.
John