We went to KD the first open weekend and I wouldn't describe it as remotely packed though all the season ticket slots were sold out. The only reason the rides had lines is they severely cut back on capacity and added in protocols which greatly lowered throughput on rides - 60-75% by my guess.
The issue KD is going to have, along with BGW, is that their customer base has just spent a year not visiting the parks and has developed new habits and activities they do that aren't KD. Getting those customers to come back involves more than just opening the gates. It means a lot of marketing, promotions, and other actions to get people back in the park, and new rides help in that department. Hershey's took a good 2-3 months to get back to a more normal attendance and it involved a lot of marketing and sales to do it. KD and BGW are going to have to go through the same pains to get their customer bases back so opening a new ride up is a good tool in that process.
I would say the counterpoint is that there may be pent-up demand for the parks in a (somewhat) post-COVID world. It's been a year since anyone has gotten the full experience at KD or BGW, and there are probably lots of people who are afraid to visit the parks at all right now. If a vaccine is widely enough available by the summer season, I think the mere fact that the parks are open again will be enough to drive people to the park. Plus, the parks' marketing departments can certainly devise a campaign to drum up excitement for the parks' reopening.
Also, like I was saying earlier, even if KD doesn't open the coaster next year, they still have a brand new section of their water park to incentivize people to visit.