PSG has a good team right now, but that won't last forever. If they don't act now, soon they will have to start building another team. Besides, their owner seems fixated on a champions league, but for that, they're going to need a player who can make the difference. That player may well be Neymar, who is pretty much third best in the world right now and in two years he'll likely be second best only behind Messi.
It's a shit ton of money, yes, but If anyone can pay it without it making a dent on their finances it's PSG.
Nevertheless, it is a huge gamble, and it doesn't necessarily mean they'll win it. Just having the best players won't cut it.
You're right, they PSG do have a good team and maybe Neymar will get them over them hump and finally win a Champions League title. If I was Neymar, I would stay at Barca and continue to play alongside with Messi.