I'm a big fan of upgradable hardware, but lately I've found that the bigger problem with Android phones is the lack of software support. I had my last phone for 5 years and finally upgraded not because there were any major hardware problems, but because the android version was so far out of date that I was starting to feel the pain of missing out on some major improvements, plus some apps actually were starting to break. I picked my current phone specifically because Samsung was promising to support four major version upgrades which is, unfortunately, industry leading among Android OEMs despite lagging hugely behind Apple's software support for their older models.
Fairphone seems to have a mixed track record on this. According to their website the Fairphone 2 got 5 major updates (great!). But the Fairphone 3 got only one update (bad). And the fairphone 4 has received one update so far with a second one promised. After that they say that they'll try to provide two more updates, but they're not making any promises because the processor will be out of support with Qualcomm by then.
This is, unfortunately, a very understandable position to take. The fact that Android OEMs rely on third parties like Qualcomm to design and support their processors is definitely the major problem here. Big guys like Samsung and Google can throw their weight around and squeeze a year or two of extra support out. But for small players like fairphone it's not surprising that they find themselves in this position.
The fact is that any sane company would prefer to make money selling new chips, rather than spending it to support old ones. This problem will persist until consumers start demanding longer software support on their devices and making it a major part of their buying decision.