This is a very narrow view of what antitrust is all about. Are monopolies essentially fine as long as they don't abuse their power too much?
I think it's more the issue that compatibility extremely difficult in technology in a lot of important ways that makes competition and regulation hard. But more importantly, at least on the internet/search/social media side of things, because it is extremely pervasive and holds huge influence on the population and has a lot of data on the population, which makes it an incredible tool to influence political opinion, so it has become very corrupt.
Historically, in the US, just about all forms of antitrust behavior (resale price maintenance, group boycotts, tying, etc) were considered per-se violations of law. That is, you didn't have to prove it harmed anyone, only that it happened.
There were a few things analyzed under the rule of reason (basically does it do more harm than good to consumers, only considering economic effects).
Since the early 1970's, that has slowly changed, and the number of things considered per-se violations has narrowed considerably. Most things are now analyzed under the rule of reason, and you have to show they really are harming consumers.
So to circle back, being a monopoly in the US is fine[1] on its own.
This is all very different than Europe.
[1] Here's what the DOJ's antitrust division says:
Unilateral conduct is outside the purview of section 2 unless the actor possesses monopoly power or is likely to achieve it.
The mere possession or exercise of monopoly power is not an offense; the law addresses only the anticompetitive acquisition or maintenance of such power (and certain related attempts).
Mere harm to competitors--without harm to the competitive process--does not violate section 2.
Competitive and exclusionary conduct can look alike--indeed, the same conduct can have both beneficial and exclusionary effects--making it hard to distinguish conduct that should be deemed unlawful from conduct that should not.
Because competitive and exclusionary conduct often look alike, courts and enforcers need to be concerned with both underdeterrence and overdeterrence.