You don't actually pay a lot. You pay what is fair. The only reason other phones cost less, that is, cheaper is, precisely, because of all the shortcuts these corporations unethically took.

For example: a burger shop can offer you a $4.99 burger, or the same burger at $0.49 with stolen meat processed by underpaid employees working 7 days a week for 12 hours. You get used to the $0.49 deal, then you start seeing the $4.99 burger as "very expensive."

The Fairphone is faster - although perhaps not by a huge amount, has better battery life and a better camera. The official spare parts shop has parts for the Fairphone 3 onwards, earlier models' parts are sold out. The Fairphone 2 was released in 2015 and has stopped receiving software updates this year, the 3 came out in 2019. 8 years of support and replacement parts seems to me to be much better than what Samsung or Google would offer you.

Specs differences: https://www.gsmarena.com/compare.php3?&idPhone1=12540&idPhone2=9536#diff-,,*

Chipset comparison: https://versus.com/en/qualcomm-qcm6490-vs-samsung-exynos-9820

Spare parts shop: https://shop.fairphone.com/shop/category/spare-parts-4?ref=header

What is the reason your S10 is on it's last legs? If you see yourself needing a lot more power, maybe the FP5 isn't the best bet for you because you're right, it might not be useful to you in the later years of it's life. However, I'm not sure what people are doing with the current top-of-the-line SoCs to justify that much power (if people want it to play games or for non-practical reasons, that's fine too). As an S10e user, I feel like we've come to a point where my current phone should be powerful enough for the foreseeable future. My only issues are I'm no longer getting official software updates (I'll probably go with a Custom ROM soon), my battery life is a bit shorter than I'd like, and parts are getting hard to come by with most being "new old stock" at best.