McDermott led the Royals to promotion to the Barclays Premier League last season but was sacked on Monday with his side second from bottom.
Nigel Adkins suffered the same fate at promoted Southampton in January while in recent seasons QPR boss Neil Warnock and West Brom's Roberto Di Matteo also took their clubs up only to be axed when they struggled.
Jackett is currently the longest-serving manager in the Championship and probably has a job for as long as he keeps Millwall there.
Yet the 51-year-old, whose side were in the play-off hunt themselves earlier this term, admits paying the price for achieving promotion is part of the modern game.
"You can't be surprised, we all know that goes with the territory now," he said.
"I don't know the ins and outs of it but I do know he's done fantastically well for the club, in lots of roles over many years.
"Promotion last year was fantastic - they won 15 of their last 17 games - and they are not down and out yet. He should hold his head up high in terms of what he's done for the club.
"Perhaps he is a victim of his own success. But if you want to be a football manager you get the sack, don't you? Maybe it's harsh, but unfortunately that's the territory.
"There are two things I don't worry about. I don't worry about losing and I don't worry about the sack."