Those who want to install apps outside the App Store already have multiple choices. Those of us that prefer the Apple model only have one choice, and you want the government to force Apple to eliminate that choice.
That's what this comes down to. Not fairness or antitrust concerns. It's simply that some people and companies want to use governments to force Apple to change their software to work the way they prefer.
More to the point, it's about billion dollar corporations using governments to force trillion dollar corporations to give them a cut. The "sideloading" argument is just window dressing to give the process the appearance of grassroots support. Outside of China, not even Android users choose sideloading or even third-party app stores in any significant numbers.
What laws like the DMA will enable is for large developers to have more leverage against consumers. I think we'll see crappier terms and higher prices to deal with the increased piracy and lower discoverability.
You will still have the App Store, and very few developers if any will leave the App Store. So you can still have your only one choice
And if a better choice comes around so be it. Competition is healthy and alternative app stores would push Apple to actually improve the App Store or else they'll get left behind.
I remember back in the 90s with United States vs Microsoft that Microsoft restricted web browsers to just Internet Explorer, everyone cheered when Microsoft was forced to allow other web browsers. If that case didn't happen, we wouldn't have Chrome or Firefox. Yet when Apple does it with app distribution (as well as web engines) suddenly it's not okay.
Most emulators are legal. Most people use them for piracy. It isn't a grey area. It's simply an excuse to justify taking advantage of the fact suing individuals for infringement isn't worth the effort.
To be clear, I don't care that people do it. I just don't think it justifies forcing Apple to allow emulators.
The Google Play Store allows emulators
Steam allows emulators
The Microsoft Xbox allows emulators via Dev Mode
Apple are literally the only ones who don't for the iPhone, yet they do for the Mac.