They could still try! But you'd have options.
Take email, for example. I cannot imagine something like that coming into existence today.
I can use my own client to avoid ads and tracking from my service provider—did I download this message? Sure, the server knows that. How long have I looked at the message? Which message did I look at next? Did I follow any links (yes, someone might track that part, but my email provider's going to have a hard time doing that)? What mouse movements did I make while looking at it? No such luck there, and yes websites and closed-platform services do track that stuff.
I can switch providers. Say my email provider starts injecting trackers into all links. I can just dump their ass if I don't like it. I keep using email, and now they receive zero info about me (I mean, they might get a little if I send emails to their users, but you get my point). If I have my own domain name I don't even need to tell anyone I switched.
I can email someone using a different provider. Yes blocklists or whatever might cause a problem but, fundamentally, this does work.
Protocols force providers to act like a telco, at least, except that the situation's even better for software because the barriers to entry in the market are so low... unless all your competitors are giving away access to their strictly closed ecosystem for free, and not supporting open protocols. Then you're screwed, and that's exactly what's happening now and why the Internet protocols are largely frozen in time.