And unless your particular battery module is a commodity component, this is worth jack.
Nokia had the right idea on this ; most of their user-replaceable battery modules had lifespans longer than the line of phones they first appeared in. But with the trend toward integrated batteries, everyone started designing special-purpose units for one particular model, which is no longer worth manufacturing after that model became obsolescent. Which is a shame, because treated well, most phones will, as you note, outlast their battery.
Batteries degrade even on the shelf when you don't use them, so unless your phone was so mega-popular that it's worth manufacturing new ones (basically just iPhones), any module you're going to get is going to be old stock, or a knock-off manufactured so cheaply that you could almost rely on being able to use your phone as a grenade.
I have a Nexus 4 that I've had 5 years, has survived being repeatedly upgraded to the latest version of Android even when Google / LG stopped supporting it, is even outlasting the TPU case I bought for it, has only one or two imperceptible scratches on the screen. It's still my daily driver and still a great phone - the only two components I worry about are the USB port (I got a wireless charger to help reduce the number of cycles) and the battery - I still get 2 days of standby time if I'm careful, but it's starting to drop charge a little quickly once it drops below 40% now. I'd love to get a brand new, official, manufactured-this-year LG battery module for it, but such a thing does not seem to exist. And even the third-party knock-offs all seem to have been made in 2013 and sat in some superheated SE Asian warehouse since then, degrading to the point where they are sometimes worse than the unit they are replacing (as far as anecdotes on /r/nexus4 seem to reflect.)