No.
If Van Gaal fecks up royally and finishes outside the top 3 (yes, three) in his last season, we shouldn't appoint Giggs anyway - I think even
@Walrus admitted that - we should go for someone proven.
If Van Gaal does well, leaves good foundations, a well-balanced squad then why the hell would we accept Giggs taking the team backwards after it?
Look at it this way: if Van Gaal had done worse last season than Moyes, would we have kept him? Even if things had improved in the second half of the season? Would we feck, he would have been out on his arse.