A look back at the past shows many players weren't successful the first time around."Form" doesn't necessarily mean much but equally I think people have a tendency to write off batsmen based on past performances when there's evidence they may have improved. Bairstow is what, 25? A lot of highly successful international batsmen have poor periods early in their career and look to have been found out in one way or another and come back a while later with a much better idea of how to play to their strengths. Sometimes it's a technique overhaul like Clarke and sometimes its just experience etc. If Bairstow is making a ton of runs and there's clear opportunities to bring him in for an underperforming player, he's worth another shot IMO. If he was 35 or this had happened several times in the past it might be a bit different.
I think form not mattering applies more when a player is a known quantity. Weight of runs in domestic cricket could be evidence that someone is legitimately better than before.
Matthew Hayden only established himself at the third attempt. Same with Justin Langer.