They don't have to make it illegal. They can just create all kinds of barriers like only allowing government approved OSes for essential services, and then using custom software can become grounds for suspicion and subject you to searches, etc.

I'm certain this is the direction we are all heading, unfortunately.

Governments will sanction the major proprietary OSes and compel Apple, Google, Microsoft to participate in their surveillance programs, and those will have remote integrity attestation and will be the only hardware and software you will be able to use to access essential services and the internet as whole, most likely.

The usage of alternative software won't be outright illegal, but will get you on a watchlist. Like you said, they don't need to make other software illegal, just make circumventing the blocks illegal.

They can't arrest everyone, but, it's one more gray area thing that can and will be used against you should the government ever decide they have a bone to pick with you specifically so you can get away with it for a long time, until suddenly you don't.