Think of the amount of publicity that there was in the run-up to the Digital Switchover for television; it went on for months, if not a few years before the analogue TV signals were turned off. The switch to digital telephony is just as important, yet it's happening now without the same amount of media coverage
Assuming that the switchover is in 2025 we're at least 3 years out, that's still a reasonable amount of time to allow for 2 years/18 months of increased publicity.
It's also going to impact far fewer people than the TV switchover did. As there's already loads of people who don't use a landline at all.
Personally having a VOIP "landline" with an app for use on a mobile/PC would be much more useful than the current setup.
Given the costs of 4G home routers (where I am the current cost is £14/month for 11mb/s and unlimited data vs the circa 26mb/s I currently get over a fixed line) there's likely to be some who opt to ditch a physical line altogether as the 4G router can just be plugged in wherever they are and they maintain the same internet as they've got. 5G is likely to increase the costs a bit in the short to medium term but allow a speed upgrade.
As to old people not having mobile phones, all my family do (and some of them are in their 90's) and have done so for a significant amount of time, in fact of the 3 family (4 really, but 2 are married to each other) over 80 two have internet and in part the reason I'm saying that the third doesn't is they are in a care home.
To be fair my dad (only just into his 70's) was a fairly early adopter of the internet due to his work involvement (science based and an international company), which means that he's probably more able to do some tech stuff than I am! Which has meant that he's encouraged and helped his parents/in-laws to have tech.
Obviously he's an outlier, however I'd suggest that there's still many who would have internet access and far more who have access to mobile phones. I'd also suggest that the various lockdowns have pushed many more to move over to having internet access.