While I agree with parts of your argument, land lines are expensive more because they have millions of miles of physical wires to maintain. Cell towers do not have this burden.
Also, Cell phone service for any smart phone is MUCH more expensive than landlines now if you are single. It's sort of like "$100 for 4" or "$100 for 1".
That said, I use smartjack (flawlessly) over my internet. $19 a year. It's mainly a backup to find my cell phone and for extremely long gaming calls (can't get one player to use skype). I think the network effect for land lines is collapsing.
Pretty soon it will be smarter to have a "land line" format phone that actually connects to a local cell tower (no lines to maintain, install, etc.).
But it occurs to me that as long as they have DSL cable service, the lines will be there anyway. So maybe the network effect won't be lost. not sure. I haven't been a landline customer for 3 years.