Take Microsoft anti-trust/DOJ speed bumps in the 90s due to Windows, because they were slowed down slightly it allowed Google, Apple, Amazon and others to crop up to compete. I'd argue we got something out of the Microsoft monopoly (desktop + internet prevalence), something out of the first AT&T monopoly (telephone lines, software, Bell Labs, C++, modern applications) and more. When they are slowed down the competition also brings benefits in Apple + Google for mobile for instance and Google + Amazon for cloud computing. Microsoft became a competitor again and it works better for consumers and developers now.
It is definitely time to put some speed bumps on AT&T unless they want to actually increase network capabilities rather than slow them down. ISPs need to be innovative and expanding networks otherwise more competition should be added. A truly competitive ISP would not be so anti net neutrality, they would win with product improvements.
Monopolies or big fish can be beneficial, until they start limiting other competitive threats through use of their power/size rather than better products and consumer benefits. When they use their size to bring benefits they bide their time and deserve the top slot and privileged market position. When they get lazy/anti-innovative and they use their size to hold back others, then it is time to hold them back a bit to increase competition.