It's outdated tech, my landline doesn't work "directly" since I last switched provider, you have to plug a phone into the router. Although, I don't bother, as I don't use landline calls at all.
In terms of powercuts... you can still make calls via a mobile phone in a power cut. If it's wider scale you sometimes lose 3/4/5g but usually not calls and texts. As for the "not everyone has a mobile" argument ... ...
for those who are massively concerned about their granny in a powercut, you can get a basic mobile for £10.