Why would ISPs care to throttle anything if bandwidth usage wasn't an issue? ISPs don't just needlessly hate Netflix and Youtube. They hate that there is not enough bandwidth at certain times of the day, which causes users to experience disruptions in their service. If Netflix and Youtube were to disappear, there would be enough bandwidth. The type of content is the reason these issues are arising, i.e. 4K and 2K streaming.
And ISPs were continuously updating their infrastructure long before net neutrality arose.
I think a lot of people are ignorant of what it takes to run the internet, and thus don't even have a drop of sympathy for ISPs. ISPs aren't these assholes who want to fuck you over from watching youtube videos because they hate you.
There's a reason there is such a huge barrier of entry into the market, running an ISP is not easy, especially with the amount of scaling they have been forced to do in very recent years.