This wouldn't happen if those ISPs didn't have local monopolies. Networks should be opened by selling traffic wholesale to other companies so that they can compete for subscribers on those networks. The network owners would have more than enough money for upgrades and if they don't, downlevel ISPs will sue them.
I'm not making a claim about the veracity of the counter argument to this, but it is easy to state. If network providers had to sell traffic wholesale to other companies, they would have less incentive to upgrade them as those networks then become pure commodities. Right now the local monopolies can use their networks to sell high margin services (cable) bundled with low margin services (internet traffic). If they ha…
Pit DSL against cable, as has happened in the UK. The POTS network is owned by one company (BT) and the cable network by others. Retail customers generally have a choice of connecting to the Internet by either. BT (POTS network owner) are required to sell traffic wholesale to competitor ISPs (who then buy their own transit). This seems to work very well here, and DSL upgrades continue.