It’s not the lack of support from the FP devs but the lack of support from Mediatek. The FP devs are doing all they can to keep the FP1 up to date as you can read here.
You misunderstood the mail. It said “you may be approaching end of life cycle”. The mail is not trying to warn you about some planned obsolesce that will be happening soon, but simply tries to prepare you for the inevitable that is sure to come some time (maybe in 5 years ;)).
Your suggesting might be a good idea if the software side was the only thing that was keeping the FP1 from being future-proof. The other issue is that FP is unable to acquire new spare parts for the FP1 as producing in such small quantities as they’d need is neither economical nor ecological.
So would you really think a lot of people would pay a yearly subscription to keep their phone’s software up to date if they know that any hardware damage would mean they can’t save the phone anymore?
Also because of the lack of new spare parts (especially main boards and screens) the number of FP1 users decreases constantly so you’d have to pay more and more each year per person to keep this going.