Yes. And I'm saying that's not really a valid comparison, because those phones are generally just monolithic slabs that have been glued shut, whereas Fairphone has to implement a user-serviceable modular design with actual seals and stuff.

Would giving it both water-sealing and a headphone jack be worth increasing the price by another €20, because it adds a new potential ingress point that the rest of the phone might have to be redesigned around? What if the jack is one of the biggest parts that isn't replacable? Fairphone 5's apparently only rated IP55, while Fairphone 4's only IP54. That's barely even really "waterproof", but more like "splash-proof". Would adding another hole in the frame be worth possibly reducing that rating to IP53 or IP52 ("drip-proof")? Would it be worth reducing the warranty by 4 years, because some amount of dust and moisture still works its way in over time no matter how robust the rest of the phone is?

Personally I think I would probably rather have the jack even if it meant no waterproofing at all. But that might not be the direction the market is leaning in, and we don't know what tradeoffs exactly they've considered to arrive at their final design with decent-ish waterproofing and good reparability but no headphone jack.

They have written about this directly in some detail, it seems. If nothing else, it shows that they have put some thought into the issue, and they're aware that removing the headphone jack will be disappointing for some users, but overall they see making the phone thinner and adding IP rating as being the higher priority:

https://support.fairphone.com/hc/en-us/articles/9836188988049-Audio-Jack-3-5mm