In economics, a well-functioning market is a market where no single actor can manipulate prices all by itself. That's why monopolies (oligopolies - monopsonies) are fundamentally bad.
In some situations where a monopoly makes sense (there is no sense in having 2 autonomous rail systems for instance), typically for infrastructures, there are strong provisions to be applied to keep the monopoly in control. Historically, these monopolies were often simply managed by a publicly-owned company (USPS, or interstate highways, for instance).
As I said in another comment, there is no rational way to defend or explain away monopolies. If you're a free-market capitalist, well you want a functioning free market, thus no monopolies. If you're a socialist, you want them publicly-owned and democratically controlled.
Large powerful monopolies are a tremendous menace to democracy, simple as that. It's not a question of their CEOs being nice people or not, it's deeply ingrained in the logic of being a monopoly.
"What about" arguments deflect attention without addressing any real issues. The closest you can get to justifying using one is to acknowledge that regulators have finite bandwidth to deal with issues and that some prioritization is needed.
But the regulators' prioritization metrics may not match your own. I don't always work on the biggest or most difficult of my household chores. Sometimes I work on ones I can complete today or which mitigate my most severe annoyances or which have a deadline for some reason.
All bad things. You should add telecom and cable as well. But here is the thing, search, social media, video, and micro broadcast messaging companies have picked political sides and have become propaganda machines. Even if one agrees with the political side they align with, smart folks should realize the dangers to our liberal traditions. The very idea of inalienable rights, open debate of difficult problems, and protection of minority rights are at risk.
Part of it is these companies should be paying WAY MORE taxes back into the system.
Having large tech companies IS a good thing because it basically leads to natural tech standards and advances things. I'll just stick with mobile area- we have iOS & Android. What if we didn't have ANY big tech companies and there were 10+ mobile OSs. Sound great for competition? Yeah I guess. But in reality, it would never have allowed mobile to take off. How do you put resources into developing an app when you have to make it work on 10+ OSs? All with their different quirks? It's hard enough with 2.
So I think there are a lot of advantages of a small number of large tech firms.. HOWEVER. When they are able to accumulate 100+ BILLION DOLLARS it can only be clear that they are NOT contributing back into the system to "pay" for their quasi-monopolies.
Large tech companies/the oligopoly we have now erodes standards because there are vendors with significant enough market power that they can make break-away standards, hurting the overall effort.
In all cases monopolies are bad. If you see it from a socialist POV, a monopoly should only be public and democratically controlled; if you look at it from the capitalist/libertarian side, any actor who can manipulates a market by itself prevents this market from working, stifles competition and should be broken up.
If there was a public, open standard such as Android (without google apps), many makers from around the globe could compete by shipping their hardware and a variety of software options. It happens only partially because of Google heavy hand, but it would work perfectly well without google (as it does in India and China).
