It's not about certainty. It's about giving yourself the best chance to reach a goal in mind. Juve had already won five scudetti and made the Champions League final but it was proclaimed repeatedly that Higuain would "take us to the next level". Higuain was bought to give the best chance of winning the Champions League. It's no different to PSG. You really think they're buying Neymar to assure themselves of the Ligue and a quarter final? They're trying to go all the way.
You're right, they did say that the Higuain signing was designed to take us to the next level. Ultimately, a CL title is always a goal for Juventus. However, we did make it to the final, which was further than we had made it the season before, so that is up a level (and while we would have liked more, he was still our top scorer in CL).
However, as far as the idea of reaching the goal in mind, management have always stated that the primary goal was a 6th consecutive scudetti, and history. They have never deviated from the stated goal of that being objective #1, as far as I can recall. Never heard them say primary goal was CL in any press release.
As for PSG, at this point that club is just not comparable to us in terms of expenditures. Spending 400M+ euros on the transfer fees for 2 players is absolutely obscene. And yes, they do want the CL, but it's about much more than that for them. They lost Zlatan, didn't win the title last year, and it looked like they were in danger of losing Veratti. They looked like they were reverting to a club that might not be able to draw top players to them. Yes, they'd like to win the CL, but they also want to be a major brand in world football -- and it was starting to look like they might be taking a step back in that regard. Spending the cash on those players was at least as much about the image of the club being a top draw/marketable brand as it was about winning the CL, imo. And of course, yes, ultimately they want to win the CL like all big clubs do.
The offer of bags and bags of cash can sure make a team pretty darn desirable for players. FFP is a joke.