Before ads, the service has one clear goal - build the best product they can for their users.
After ads, the goal is less clear. They still need to please users, but they also have to please advertisers. The needs of users and advertisers aren't always going to be aligned, and so users should lose trust in the Perplexity results.
If I was an investor, this would make me nervous. Make something that is far better than your competitors and users will pay. If you make something that is only marginally better than your competitors, users are only going to pay at most, a marginal fee. Perplexity is signaling that their product is mediocre.
I have a 1 year subscription to the pro plan that I got for free. Unless it gets way better, I won't pay for the next year.
I do pay for Claude and think it's easily worth the $20 / month.