In the case of DRM, chrome is fulfilling a market desire that firefox had to follow suit with once the companies that adopted it realised their desire could be fulfilled. This is an issue with the companies implementing the DRM, not chrome saying "Yes we can meet that need that no one else is providing for". Also firefox still allows you to disable that feature, so you still have that choice not to allow DRM content.

All of this though is still based on the premise that leveraging their market influence is a bad thing, I just don't see it as an issue. If they use that influence to push a product or service that people don't like, people will not use the product and they'll have lost influence.

In the example of mail servers being "harder to maintain" because of over zealous gmail filters all I have to say is: Tough. People like not getting spam and having over zealous filters is a method of achieving that. If you want to be able to send mail to Google's mail service, you'll have to meet their rules.

I'd actually be very surprised if the filters are intentionally ruling out alternative mail because that would hurt the marketability of gmail.

In fact, that's the crux of it. That's why the legal definition doesnt match the reality of monopoly, because in order to be a monopoly you have to be providing the best on the market and if you're not, a competitor will and you stop being a monopoly.