I'd ask the inverse of the question: morally, should a single gatekeeper have the right to deny two consenting parties the ability for one to run the other's software?

Especially when that ability has been established practice and depended upon for decades? And the gate-kept device in question is many users' primary gateway to the modern world?

There's nuance here, of course - I'm not morally obliged to help you run Doom on your Tamagotchi just because you want to do so. But many people around the world rely on an Android device as their only personal computing device (and this is arguably more true for Android than it is for iOS). And to install myself as an arbiter of what code they can and cannot run, with full knowledge that I could at any time be required to leverage that capability at the behest of a government those worldwide users never agreed to be dependent on? That would be a morally fraught system for me to create.

The moral justification is that I am a citizen, and can demand the laws I want. When enough people think like me, we can actually make it a law. By holding the smartphone OS oligopoly these companies hold a lot of power on the people. I do not like that. Hence I like laws that try to change that.

> especially as engineers who make their living by creating intellectual property and probably wouldn’t want to see control of it seized randomly

If these people try to use their intellectual property to control my device and hence my ability to do things, I want to have a say what they do. Yes, that is what software is: directions to machines. I own the machine, hence I want a say what it does. You are free to keep your intellectual property for yourself, if you want to.

At some point free markets become fiction. There's no financially viable way to start competing businesses in markets as entrenched as mobile OSes. Otherwise this would have happened. And if that becomes anti consumers, then the consumers start changing the rules the companies operate under. Because in a democracy we have more consumers than CEOs,so they vote with majority.

(This obviously simplifies things, but ultimately we as humans still haven't found the one and only true philosophy or moral, and maybe that's not possible (I'm no philosopher))

There are no absolute morals. But I think in general healthy societies are arranged around the ideas that people should have: the basics of living (housing, food, vacation, and some luxory), agency, and equal opportunities.

It should be clear that having a small number of companies murder all competition and personal freedoms (like doing what you want to do with something you own like a phone) are in contrast to these basic values.

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Or the alternative, more blunt answer: it does not require a moral justification. EU citizens directly elected the EP, the EP ratified the DMA. So Google can either comply or leave the EU as a market (which they wont do because it's too large and others would be happy to take it).

They are 100% free to not sell to European customers

The moral argument is that vertically integrated monopolies threaten the rights of consumers, who are human beings. Corporations are legal fictions and their "rights" are another convenient fiction to align incentives. They carry zero moral weight.