[turquoisevar]

If Microsoft would be the one paying, then it might not lead to antitrust issues, because Microsoft doesn't have a near monopoly on the search market.

On the other hand, it might also create antitrust issues if the market definition were to be broadened.

Either way, the who is just as important as the what in cases like this. Your attempt to equate the two seems to be a fallacy

[Jensson]

The argument is that you can't ban market leaders from participating in fair auctions. Google isn't using its market dominance to force Apple to make them the default search engine, they are just paying money and aren't applying any other sort of pressure.

So my argument is that there isn't an antitrust issue here, not that Microsoft paying would lead to antitrust issues. Winning fair auctions isn't an antitrust issue. Antitrust is when you issue abusive contracts such as "You get a discount if you only sell our products and no products from out competitors", but that isn't a fair auction since you use your dominance to change the deal and pressure the vendor to stop selling others products. In this case Google just bid higher than the competitors, there is no other reason for Apple to use them as the default.

You would need to show that Google pays more than they earn for the default search engine spot, or that they used some other means to force Apple to accept the deal. But I have seen no evidence of that.

[turquoisevar]

I believe the argument counter to yours is that antitrust is more than just abusive contracts.

It also encompasses behavior facilitated by market dominance that makes it impossible or too difficult for new market entrants to compete, including behavior that, under different circumstances, would be completely fine.

A generic example of something that would be completely fine in many situations would be a merger & acquisition. It's the best example of what you describe as “winning a fair auction”. Companies B, C, and D might all make an offer to buy company A, and since company B made the highest bid, they win, and all is fine.

If, however, company B is a huge conglomerate and the M&A of company A would consolidate even more power into Company B, perhaps even increase their monopoly position to the point that company B ends up being the sole company in multiple verticals, then this might prove to be an antitrust issue even though no abusive contract is in play.

Because this can have undesirable outcomes in terms of competition with unwanted effects downstream for consumers, we, as a society, have decided to put some guardrails in place in the form of antitrust legislation.

Circling back to the situation at hand, the argument at hand by the DOJ seems to be that Google has gained a significant market leader position and that deals such as the one made with Apple make it impossible for other competitors to compete effectively. Even more so when it pertains to search engines because they seem to rely heavily on usage data to be able to improve.

At face value, that argument isn't much different than the argument behind preventing M&As that are deemed antitrust issues.

The DOJ’s arguments go further, however, in stating that it is the market leader position combined with the deep pockets funded by Google’s other divisions that make it possible for them to offer billions in the first place. Which, within the antitrust context, adds a deeper dimension beyond just the “you're big, you shouldn't get bigger” argument I mentioned above.

You seem to touch upon this a little in this part of your comment:

> You would need to show that Google pays more than they earn for the default search engine spot […] But I have seen no evidence of that.

We will be unlikely to see evidence of that, assuming it exists, because the parties and the judge would discuss that under seal, and only they will get to see those numbers (unless someone does a whoopsie and forgets to redact it before uploading it to the docket).