Dayum, that's brilliant!
FWIW, it might be better to turn off the "footers" by ticking the Footers
box on the Slide Master tab of the Ribbon. You can't do this on the Master
slide, but you can do this on each of the Slide Layouts.
I can't exactly say why I think this might be better than actually deleting
the footer and slide number placeholders from the Master slide, though. Just
call it a gut feeling, I guess.
Also FWIW, another way to make the footer/slide number placeholders
immovable on the slides is to hide them using Home tab | Select | Selection
Pane and clicking the eye icon to hide them. Trouble is, you have to do it
for each individual slide -- it doesn't help to do it on the Master slide --
and then you have to remember to go back and turn them all on again! It's
not a very good solution, but I thought I'd mention it just for the sake of
being complete.