Never before have I seen PSG as high among the bookmakers as now. Of course that has to do with Messi going to the club. They are top favourites, otherwise it would be a matter of time before they would become champions. Even without Messi.
They have spent the most money this transfers season and Messi is nearly confirmed. They will win the league and they will probably win the Champions League. PSG will be punished for overspending in a couple of years but they would only achieved their goal of champions league. They will be unstoppable for the next couple of years.
Having superstars and spending a lot of money is not the main indicator in soccer! A good example is last year's league, where Barcelona, Bayern, PSG and Real Madrid lost in 1/4 and 1/2...
PSG can undoubtedly win the domestic championship, but they might not have enough experience for the Champions League... Although I also think they are worthy of a final this year!