It doesn't need to be sustainable for everyone for it to work. They are banking on sufficient numbers adhering to it for it to achieve the required results. I think that as the sense of risk remains high (and that sense will continue rising for the foreseeable as death stack up) most people will simply be too spooked to not do it.
Also, lockdown doesn't stop you leaving the house for a walk or run, or even seeing friends/family as long as you're sensible. And we all have plenty of home comforts to keep us busy/entertained.