Looking what you write, I think you misunderstand what NN does.
The monopolies you mentioned have very little to do with NN. Someone already gave example how Facebook replaced MySpace, similarly Google, replaced Yahoo, AltaVista etc, I suppose Amazon did not replace anything, but started e-commerce (and currently creating other markets, like cloud computing), but it is getting higher and higher competition.
Apple and Microsoft existed before Internet and are just using their old position to leverage their existence, but the fact they face competition.
> EDIT: another question :) If net-neutrality has absolutely nothing to do with the tech monopolies maintaining their power position then why do they all support it? [https://internetassociation.org/]
Simple, because it expands powers of existing monopolies making them relevant. Perfect example is Netflix. They providing service to stream movies. They pay also pay large amounts of money for Internet access and ability to send content to their users. On the other side users similarly are paying for Internet access to be able to receive the content.
You would think it's double paying (and it kind of is), but it's normal, why users should subsidize Netflix costs or why Netflix should pay for users' access (since they are not only using Netflix) they just pay for their access to the network.
But, regional ISPs (in this case Comcast) started having "problems" where packets from Netflix were dropped, because "Netflix was sending too much data to them", ignoring the fact that that data Netflix was sending was requested by ISP's users who were already paying ISP to do one thing and do it well (i.e. deliver data they requested).
Things were kind of bad for the users (and users couldn't also switch their provider because at most they only had just another option which was equally as bad), until Netflix reached deal with Comcast where they pay Comcast to deliver the content (something that Comcast users are already paying for).
Net neutrality is trying to make sure this kind of manipulation is not allowed.
The thing with Netflix already happened but could easily be translated to other companies. For example (I'm borrowing it from here [1], I think you should watch it it explains the core issue very well IMO) Microsoft could pay so Bing has always fast access at cost of Google. Don't you want as a user to not have any company dictate you what you can access or not? Similarly those Internet companies also don't want to pay extra on top of what they already are paying. That's their interest. The only companies who would benefit from lack of NN are handful companies that deliver Internet to end users (especially companies like Comcast, TimeWarner/Spectrum, AT&T, Verizon etc) and lack of NN would give them huge power to bully anyone else.
[1] https://www.youtube.com/watch?v=92vuuZt7wak