Would the DCU be in trouble if Geoff Johns suddenly jumped to Marvel?

Eh. No writer/artist is indispensible (or that important). Just as if Bendis, Millar etc (any one) leaving Marvel for DC wouldn't put Marvel *in trouble* so won't Johns' changing sides.

Even if Johns is the biggest architects of the current (Post-IC) DCU, he isn't the only one...i.e. nor is he DC's ONLY writer nor the only one who can writer (good?) stories. As has been mentioned many times in this thread were Johns to leave today there would be over a dozen ready-replacements for him, quite a few of them equally good if not better than him.

As for the 'direction' of the DCU, it too would just change according to what the new writers/editors will decide for it.

Johns is the current BIG thing at DC. He wasn't a few years back and he won't be a few years from now, (how many years they might be). Some new BIG name will take his place and direct the DCU according to his *view* of the characters. Who knows just as Johns, Didio and others have swept away the Post-COIE Pre-IC DC in favor of Silver Age fanboyism, the BIG name writer/editor then could do away with all of their/current setup and go back to the previous one...or start something afresh.

I used Bendis as an example because like Johns he (Bendis) too is one of the biggest architects of the current MU-616 setup. Both writers have done MEGA events and both have multiple monthlies on their desk.

And yes, Bendis's leaving Marvel would have a greater effect than if Johns were to leave DC. Even though he might have rebooted DC's continuity, Johns' effect on overall DC isn't as far reaching/deep as Bendis is at Marvel. Also, with SI, Bendis is more in the attention of the comic reading audience, while over at DC Johns is just working on his monthly titles. Sure there is the Legion event coming, and yes it will sell well...but it doesn't quite compare to the level of SI (or even at DC, either IC or FC). The Legion has its fans and Johns' name will attact new/temporary readers but as a franchise it just isn't that big/mainstream. A year down the line (whenever the GL mega event come out) if Bendis isn't headlining a mega event of his own, the balance will shift towards Johns' side.

Yet, neither writer is so important that they are indispensible.