If you notice it's dirty, don't conclude that you have to clean.
Instead, because you're noticing, you should conclude you want it to be clean. That obviously requires you to do some cleaning, but it's something you want, not something you're forced to do.
Yeah, the rage seems to usually stem from two misunderstandings:
It forces you to use Bluetooth headphones.
A 3.5mm jack is cheap and trivial to build into a phone, so there's little or no reason to not include it.
You already pointed out why neither of those are legitimate reasons. For 1 you just need a simple dongle, not new headphones. For 2, because the Fairphone is modular and repairable, it's not just the 3.5mm jack but also a custom replaceable daughterboard they'd have to develop and keep in stock.
Not having a 3.5mm jack is a minor inconvenience at most, I don't get the rage either.
I heard this tip that works decently sometimes.
If you notice it's dirty, don't conclude that you have to clean. Instead, because you're noticing, you should conclude you want it to be clean. That obviously requires you to do some cleaning, but it's something you want, not something you're forced to do.