But the contrarian opinion would amount to forcing companies to interoperate, which is a massive endeavour of standards and committees - usually only undertaken for natural monopolies to avoid tragedy of the commons.

Are smartphones a natural monopoly? And at what levels? Hardware (including plugs and jacks, physical button locations and functions?), software, data federation? Like where do you draw the line?

If smartphones must allow alternatives, then why not gaming consoles? It's a similarily integrated device. Would Microsoft be forced to allow unlicenced 3rd party software on the Xbox?