Chessington doesn't receive that many visitors in comparison to other parks in the country; in 2018, it was the least visited of Merlin's British theme parks according to the TEA attendance report, with 1,670,000 guests visiting. By comparison, Legoland Windsor had 2,315,000 guests, Alton Towers had 2,100,000 and Thorpe Park had 1,880,000, so it's definitely the least visited Merlin park in Britain.
However, I must admit that the park is doing very well at seemingly keeping up similar guest figures to what they had in the park's initial years, so they must be doing something right! 2014 was allegedly the park's second highest year for attendance behind 1995, for example!
As for what Chessington could do long term, I actually think that re-emphasising the zoo aspect could work well for them in the long term. In recent years, zoos seem to have had somewhat of a resurgence in popularity across the world compared to how popular they were when Chessington was first turned into a theme park. It is also worth noting that Chessington struggles with planning restrictions even more than the likes of Alton Towers do; the locals have said that they do not want another ride on the scale of Vampire built at the park ever again due to noise, and Rattlesnake had to be dug into a pit at only 49ft high. For this reason, I think that Chessington should probably try and emphasise the zoo aspect of the park if it wants to remain successful while still retaining some of the rides (sort of like West Midlands Safari Park does), and they do seem to be doing this to some extent. For example, they rethemed Mystic East around tigers in 2018, and they're building a (presumably, from the concept art) crocodile-themed drop tower in 2021. As much as I'd love to see Chessington embrace the full-on theme park route, I don't think planning restrictions would allow for it nowadays, and I also think that zoos are arguably more popular than theme parks now, especially among the young family demographic that Chessington aims at.
In terms of the Rainforest area, I think it looks OK; perfectly nice for what it is. Bearing in mind that it's a low investment year just before a high investment year where the park is building a new drop tower, I think it does the job. It looks vibrant and colourful, there are some nice little themed features in places and on the whole, I think young kids will like it!