Haven't personally had a landline for years. But my grandmother's house landline costs $50 a month from AT&T. I feel internet + a cell line provides sufficient redundancy as long as equipment is in place to keep things running during a sustained loss of power.
My modem & computer are on big fat UPS device so I can browse the internet while the power is out. So in the event I lost cell service (I only recall it happening once) I would use VoIP to make calls. If a storm knocked out my cable line then odds are it took the phone lines with it since they share the same poles. But those lucky people with ground-laid fiber will probably have internet service that's even more reliable than a phone line, and have zero reason to pay for a phone line.
If the question is from a long-term power loss perspective then for the cost of a landline one could invest in a big UPS and some electronic battery packs and keep a cell + internet access going for days to a week depending on the devices accessing it. The last time we didn't have power for a >24 hour period the internet never stopped working here and I was still using it with a laptop off the UPS. $25-50 a month adds up fast, for that money I'd rather put it into two big UPS devices instead and save the rest.