You are assuming the number of people who will stop shopping because of masks won't be matched by the number who are now happy to go shopping because of masks. Given that those who are shielding are soon to get a temporary reprieve from restrictions, I think you are wrong.
The high street was suffering before this. I think the problem with masks is that it will impact the leisurely or impulse shopping that I assume brings in more money than a quick dash in for essentials. I used to mooch about for hours trying things on and maybe doing some impulse shopping and it's those shopping trips that would result in me spending a lot rather than a quick dash in for one specific thing that I can't get online.
The key thing will be whether the high street reports a drop or increase in footfall after the 24th because then it will be concrete proof of whether mask wearing increases or reduced shopping in person.