The first question asks whether we should sack Van Gaal if it meant getting Guardiola while he was gettable. For me the answer is clearly no. You don't sack a manager who's doing quite well, whiche LvG is, just because someone else is available. That's cvntish and it's the kind of stunt Chelsea would pull.
Now, if the question is whether we should bring in Guardiola when Van Gaal's contract is up, I'd go for it. He's a proven winner as a manager and has a big enough reputation to bring in top players. I can't think of any other manager I'd rather bring in. Definitely not Mourinho (although at the time I thought Mourinho would have been a fantastic successor to Ferguson), probably not Ancelotti (been there and done that too many times), definitely not Benitez (no explanation required) and not Klopp (you can't go from Liverpool to United). Simeone and Koeman would be intriguing.
There is a more pressing and difficult question we should be asking, which is whether it makes any sense to let LVG go into his last season as a "lame duck", so to speak, as Ferguson did going into a season about a decade before he actually retired. It didn't work out well for Ferguson that season and may not work out that well for Van Gaal. I don't know what the right answer is to that question. All we know right now is that Van Gaal is in the second year of his contract, he has the club looking up and not down and we'll have to cross the bridge of his future with United next summer, not now.