Argument is simple Bairstow batting vs Foakes keeping.
Bairstow is no doubt a match winner and fits the plans of what the team want it harsh but you can see the reasons, Harsh on Foakes but better keepers have been left out before because of team set up.
The argument is simple, as you rightly say. I also fear those espousing it may be as well!
I agree, Bairstow is the better batsman. Foakes is the better keeper. But to limit consideration in this way is awfully reductive. Yes, Bairstow might average x more than Foakes (or more accurately might play one truly match altering innings in a Summer) but the dismissals Foakes will make that Jonny won't more than offset that. Especially if its Labuschagne, early doors on a shirtfront. And the damage to Bairstow's ageing, not entirely svelte body of keeping damages his long term availability across three formats.
Indeed, the argument should not even be between the two - it should be a question of who the best XI players are - and I think consensus would be that both come into that category, and one Zac Crawley does not. Maybe that means Foakes is told to try opening and just give it his best shot (lets face it, he can't do worse than the new Liebenburg!), maybe it means someone else is.