Note that the viewpoint that antitrust is SOLELY about benefit to consumers, and not also about restricting corporate power, is relatively new in US antitrust jurisprudence (around the 80s I think), and there are currently many scholars reassessing this viewpoint.
Edit: Some more info about current reassessment of the Chicago School of Antitrust: https://www.law.uchicago.edu/news/reassessing-chicago-school...
Google is in a position similar to being a major broadcaster- it is a gatekeeper of content, whether through search, YouTube, or ads. While Google started off with a pretty radical view of free speech that resulted on a pretty content, that has waned in the last 5-10 years.
The most obvious of this is the de-monetization of YouTube channels. While I hardly disagree with their choice to distance themselves from many vitriolic personalities, I've also seen them demonetize science channels for doing experiments involving explosives and smaller channels they just didn't want to bother monitoring. The theoretical reason for this- their ad partners don't want their ads accidently running on offensive content.*
There isn't any real alternative to YouTube. Searching for how to videos on google often results in a page of only YouTube videos. Content producers cannot directly negotiate interstitial ad content. The old-time concerns about a single broadcaster dominating the marketplace of ideas is absolutely applicable here. Breaking up Content, Search, and Ads would go a long way to ensure access to the platform.
*As well they should, and I actually kind of LIKE that its become harder for offensive crazies to make money, but I'm providing the argument for breaking up the content, search, and ads.
There used to be (and might still be) limits on how much national advertising national networks could require local stations to carry, on the theory this made local stations more independent (and in tune with local needs). This is my 21st century version of that regulation.
Like I said, I'm not even sure I'm in favor of such a break up, but I can see the historical parallels.
Commodity monopolies on the other hand seem to have much more obvious detrimental effects longterm.
"Amazon is eating into Google's most important business: Search advertising" - October 2019 [2]
There are dozens and dozens of articles about this. The market as a whole is growing, and Google's market share is large, but shrinking as a percentage of the pie.
[1] - https://www.investopedia.com/news/facebook-google-digital-ad...
[2] - https://www.cnbc.com/2019/10/15/amazon-is-eating-into-google...
If you were to impair the ad business of Google, Facebook, and Amazon, wouldn't that simply shift profits to Verizon, Microsoft, Snap, ByteDance, etc?
If I want to advertise my vacation house for rent, I sure as hell am not going to do it on Amazon, and FB would hardly make any sense (maybe if I had a whole network of houses for rent to do brand marketing, but not if I just had one). Google is pretty much the only option that makes sense here.
I mean, just ask a digital marketer. While there are some potential areas of overlap they all view Adwords, FB ads and Amazon ads very differently and not much in 'competition'.
I imagine other large players in vacation rental marketing are VRBO, TripAdvisor, and AirBnB.
Anyway if you redefine the online advertising market to include only the things that Google dominates, such as search ads on Google itself, then sure you can make it look anyway you want.
And look at your "other large players in the vacation rental market". Perhaps unsurprisingly some of those travel brands are among the largest spenders on AdWords. See Expedia (parent co of VRBO) Chairman Barry Diller's comments on this exact topic: https://skift.com/2018/10/16/expedias-barry-diller-calls-on-...
If you look at search marketing, which is unique in its ability to target users at the moment of intent, let's look at the players. Google is at over 90% and everyone else is peanuts. I would agree with you that there is overlap with Amazon here, but only in a subset of products (i.e. primarily physical, deliverable products, not services).