I think it heavily depends on each user group and their reason for looking into UPS in the first place.
I personally wouldn't be impacted much by a power outage normally, since I can just use my mobile connection. And that could be shared with anyone in the household who might need it in such a situation. So under normal circumstances, not even I would have seriously looked for battery and protection for a hard line connection like FTTP for internet or VOIP.
However, since my parent had a medical issue last year, and I became a full time (unpaid) carer, I basically had to look for all sorts of backup options in case mobile connections go out or they get swamped when everyone starts using them; I've had o2 engineers accidentally cut a cable back in 2005/2006 that killed their o2 mobile for hours on end (not even emergency call access), at that point we had two mobiles with o2, and swore off from staying with just one provider from that point onwards to mitigate any chance of something like that happening again.
So I began looking into UPS backup in case mobiles aren't working (or get swamped), and of course, if the national grid ever ran into the threat of needing to do rolling blackouts. So, I went looking for something which could power the comms equipment at my parents place for 3+ hours. Thankfully was able to find the Cyberpower 1500va Sinewave UPS, which had a battery (swappable) just about big enough for just over 3 hours, so I'm no longer worried about rolling blackouts impacting my parents place either, nor surges burning anything out in the comms section (ONT/Modem, Router, Landline DECT Phone, VOIP box).
If anything, I'm more worried about how long our providers backup power will last if a rolling blackout hits our area. Ours is tested to last the 3 hours of blackout, but I'm unsure how much battery backup their local connection system has. So we might be up, but our connecting point to their network might not be up for all 3 hours.
But yeah, it wouldn't impact me and the extra expense for a large UPS that can outlast a rolling blackout for 3 hours would put me off from grabbing one. However, circumstances have pushed me into researching, considering and grabbing one. So, as said, it all depends on each user group and their reasons for looking for a UPS.
As for getting a replacement ONT, assuming it's not from something you obviously did to damage it, I would assume they would replace it for free. But no personal experience with that, so can't say for certain. Someone with more experience with BT FTTP might be able to offer you more accurate insight into the matter.