Please tell me, what you would consider to be the frame in this case.
In my opinion the SoC/Motherboard is to the phone what the frame is to the bike. You already get better cameras, a better display, a new battery and maybe in the future, the other modules might get an enhancement as well.
And the phone is open to customization as well. You can get or design your own cover, someone else has already made an extension plus a new cover to house it:
The one thing, that’s different to buying parts for a bike, is the fact, that you get no choice. If you need a new camera, there is only the new one (unless you buy second-hand of course). Fairphone has not the means to offer various choices; not to mention, that the difference - pricewise - would not be that much, that anyone would go for the lower quality option.
Still, in all, I would agree with @ben about getting a FP2 now, if you want to keep it for 5 years.