The departure of Messi was not solely orchestrated by Nasser Al-Khelaifi, the player wanted to leave. And this fact showed in his behavior on and off the pitch. He traveled to Saudi Arabia without permission. I guess the interest was getting from clubs which include Inter Miami, Barcelona, and Al Hilal was attractive. The fact that his children were not registered for resumption was a sign that he didn't want to remain in PSG. I also think that the failure of the team to win the Champions League with Messi also spurred the club to let him go. In the case of Neymar, his injury problems are becoming a challenge and was looking as if the club was wasting funds on the Brazilian without him performing. And the fans are becoming impatient and want him to leave by all means.
The club is really missing Messi because he is creative and also a goalscorer. Dembele wasted a lot of opportunities that Messi would have easily converted. But the game against Nice shouldn't be used to generalize PSG's chances of winning the league because Nice is also a good side. The outcome is the game is showing that other teams are coming up to compete and stop PSG from cheaply winning the league. The Ligue One title is not their birthright, so I am happy that some teams are giving the defending champions a good fight.
A tempting offer from Inter Miami made Messi leave, even though on the one hand PSG club owner Nasser Al-Khelaifi wanted to keep him. Talking about performance, it is also quite realistic if PSG releases Messi because he is 36 years old. That's not a young age to appear regularly in PSG main squad, it's hard to see Messi becoming a reserve player at the club, so it's as simple as that I think. For Neymar, I agree with your statement, if Neymar hadn't been injured so often, PSG probably wouldn't have let him go last summer.
The results of PSG transfer market movements have not fully contributed, the defeat to Nice is proof of that. Luis Enrique has to rack his brains to find the right tactics for his squad, otherwise PSG will have difficulty in Ligue 1 and UCL this season. I think the first match in the group phase will be a benchmark, whether they are strong enough in the UCL this season or not, what is clear is that Dortmund is stronger than Nice.