As a child in the 50's, I remember answering my grandmother's landline phone (I still remember her telephone number). Basically, the same analogue technology (using copper wires, which also carry power to maintain a phone call to the house) has not changed. It is has become more complex and difficult to maintain, and a bit like trying to keep a vintage/classic car running in modern traffic, the telecoms systems, infrastructure and engineers have all moved on.
I recently upgraded from the old copper line to full fibre to the home. I have kept my landline (two new handsets, one upstairs, the other downstairs) too. Both DH and I also have modern mobiles. I have a smartwatch that works with my mobile (no need to carry the mobile around the house).
We receive many landline calls and feel we would cut off many of our callers if we opted for mobiles only.
If you change to VOIP with a new router/hub, you won't need to change your old landline handsets. However, if they are currently wired and plugged into a socket, you will need to plug each one directly into a Digital Voice Adapter (DVA - one DVA to each handset) which will need to be in a power socket. Each DVA will need to be paired with your router before moving them close to your handset. Your telecoms provider should provide you with additional DVAs if you need them.