But with the collective meltdown around the Rose news the situation was especially bad for them. Dortmund could sell Sancho for a decent fee, had a huge credit line and eventually issued some new shares to the tune of nearly €100m, Leipzig and Leverkusen just called up the marketing division of their parent company, Bayern is Bayern, everyone else got a bit fecked. He had a bunch of players he needed to sell, but at the same time they nuked their own value and every else's budgets got cut. If I remember correctly they actually were about to sell Thuram to Serie A, but then he tore something just before a transfer could happen. I think (arguably) in normal times they would have just been able to extend Ginter and sell Zakaria and maybe Plea/Thuram.
What do you mean by "he should have taken responsibility", isn't stepping down, admitting that you messed up and drawing a line under that fact taking "the L"/responsibility? He was with the club for 16 years, I think that merits a press conference at the end.
It's also not as easy as just saying "I'm back". He probably burned some bridges in his final months at Gladbach and more importantly his job has been filled with someone else, who implemented a new strategy in the mean time - Gladbach themselves probably wouldn't/shouldn't have any inclination to just sack Virkus and give the job back to someone, who got the club in deep trouble.
