Possibly.
If there are bugs in the hardware drivers (especially concerning security) which the hardware vendors (especially Qualcomm) would have to fix, but don’t anymore, then yes, this would be a problem.
But it would be a problem I guess many devices supported by the LineageOS community share, because many of those devices don’t receive any update of any kind anymore officially, LineageOS at least covers the OS side of things for them.
I think so.
My understanding up to now is:
First and foremost the hardware vendors have to patch their stuff if some bug comes up, as long as the stuff is not EOL.
There’s just not much Fairphone can do about it other than wait for updates to be delivered by the hardware vendors … unless the hardware is EOL and the vendors give Fairphone (read as: sell, if Fairphone wanted to have it and could afford it ) the source code to support it any further.
And then Fairphone would still need to turn this into updates when necessary. This is not trivial, they tried something similar and it didn’t quite work out in the end (even if they came close) for Android 4.4 on the Fairphone 1.