It might be a strong argument depending on the negative effects - I don't think it's very clear cut. Also no, neither Apple nor Google have a duopoly on the distribution of all consumer software. Microsoft exists, for example.

The other problem consideration here is negotiating power.

Today consumers don't have negotiating power over individual developers, but both Apple and Google do. If you complain to Meta about their unwanted tracking, you don't really have many options besides deleting the app (which you should do anyway). But if enough people complain to Apple or Google, they are more inclined to do something and have the power.

While it may be a marriage of convenience, it's undeniable that both companies through their app distribution models have also provided benefits to consumers that developers otherwise would have abused - privacy, screen recording, malicious advertising, &c.

If you want to argue from the standpoint of pro-consumer action, you have to remember that "developers" are usually pretty awful too and will get away with anything they can, even if it harms their customers. A good balance, instead of ideological purity about one "side" or the other is the smarter move. I tend to come down on the side of the mainstream app stores precisely because those asking for more "freedom" to do as they wish are a tiny minority and are usually more technical. I.e. they can jump through the hoops to install 3rd party app stores and jailbreak their phones today, and since you already can do what you want, maybe it's best to just leave the masses alone since they're very obviously happy with the duopoly.

There is nothing stopping you from using non-Apple hardware to escape restrictions on downloading unreviewed software.