Part of learning self-discipline is learning what triggers or facilitates undesired behavior and modifying one's environment to increase desired behavior. This can include limiting internet access, such as removing home internet, or switching to a dumb phone. This isn't weakness; it's learning about oneself and working with what you have. Certainly, there are ways to increase will-power, but relying purely on will-power can be difficult for many people.
I didn't say it was "weakness". I meant it seemed like unnecessarily crippling oneself. It might be valuable for many people to investigate why they can't stop themselves from overusing their smartphones. Sure if they end up realizing that they can't manage to stop themselves, it would make sense to change the environment, but why give up without even trying? This really seems like the opposite of learning self-discipline.
I didn't mean to imply that you said it was a weakness, though I did think some may consider it as such ("what, you don't have enough self-control to limit your smartphone usage?"). I'm addressing the fact that self-control is not limitless (even if developed and increased) and that if one's goal is to be more productive, one should feel free to use the tools available to them, which include both modifying one's environment and increasing self-control. And of course any modifications have to fit in with how you want to live your life. If it's crippling, it's not a useful modification. Having a dumbphone may be crippling to some, and liberating to others.
You seem to have dismissed the former entirely with "that doesn't seem sensible to me at all", and here leave it as only a last resort. I don't believe that's a useful way to view it. Use the tools you have available to modify your behavior.
You don't need "infinite" willpower. You might even need very little once you've changed into a default mode of using your phone less. By the way it's still unclear whether ego-depletion is a real phenomenon AFAIK.
I'm noticing that for me, simply disrupting the pattern does wonders. A background process that re-bans websites via localhost redirect in my hosts file every few hours does wonders, for example.
A bad habit I can fall into as easily as breathing is a problem.
A bad habit I have to indulge in some effort to perform, puts me in the position of needing willpower to indulge in it!