If not for regulatory barriers, do you really think Bell Systems would ever have lost its monopoly over US telecoms?

It's always hard to talk about history that didn't transpire but yes, due to technological change similar to IBM.

But they controlled all the phone lines, and hence all the broadband – they'd only be ousted when 3G came along, and even then would they really have?

Sure regulatory capture can entrench interests, but there's no reason to believe that in the absence of regulation one big company wouldn't aggregate every major market and just destroy all competition.