Comparing these two palters is not right but I think Dembele can have much better performance in PSG than Neymar during the time when he was playing for this team, especially because we know Dembele left Barcelona because Xavi didn't believe his skills at all and this player is even more motivated to have better performance to prove himself. Also considering the age of Neymar and Dembele, we know Dembele can be a good investment for PSG in the next years.
PSG have good players that would surpassed the actions and performance of the former ones. Ousmane Dembele and Neymar Jr are almost the same pairing interms of physiques and crucial abilities on the pitch. PSG have troubles with Neymar Jr because he was inconsistent in league games, always vulnerable to injuries that would ruled him out for months and missing half of the league. The parisians boss, Luis Enrique will make things work a little bit faster for the Frenchman, he knows basically how to utilized Dembele's performance to blend with his ultimate objectives.
Why are you then saying that the new players will surpass the performances of the former ones when in the next sentence you write that Dembele is almost the same as Neymar? Which is actually true because Dembele is as susceptible to massive injuries as Neymar and misses most of the games. You can check his injury history and will see that substituting Neymar with Dembele is really risky because it could be exactly the same problem. Replacing Messi is impossible as well or who do you think will play as efficient as Messi did?