I think your suggestion is kinda based, but it's definitely not more simple, as it would require targeting a single company and enforcing a monopoly action against it to force it to split up. I believe the last time that happened in the USA was 1982 when Bell System was broken up: https://en.wikipedia.org/wiki/Breakup_of_the_Bell_System
Meanwhile a legal mandate to allow third party app stores can be accomplished more simply (imo) similar to how the EU forced Apple to use USB-C: market directives.
Let's be honest, there's probably a dozen antitrust actions that are overdue in the US tech sector, and since the US doj has ignored that issue for so long, it now probably needs to be done the Standard Oil way, with a hacksaw. Of course it's not going to be easy, but the current situation happens because for 4 decades the US has taken the easy way, or done nothing at all.
As for simplicity, it lies in the fact that lighter actions often don't produce enough effect, can be side-stepped or simply ignored without continuous oversight. On the other hand, when you force divestment and bust corporate control, it is much easier to guarantee that the desired effect happens.
I think your suggestion is kinda based, but it's definitely not more simple, as it would require targeting a single company and enforcing a monopoly action against it to force it to split up. I believe the last time that happened in the USA was 1982 when Bell System was broken up: https://en.wikipedia.org/wiki/Breakup_of_the_Bell_System
Meanwhile a legal mandate to allow third party app stores can be accomplished more simply (imo) similar to how the EU forced Apple to use USB-C: market directives.