I personally think that we will see investment before 2024, even if it does end up being more minor investment. Banks that offer loans and such to fund these sorts of investments will know that COVID-19 is only a temporary disruption to business while lockdown is in place, and investment is what gets people into theme parks, after all. Also, dependant on what sort of insurance Merlin has, they're probably covered for while the parks are closed.
I know that there is also the question of the effects that the likely global recession will have on attendance, but the parks seem to have been somewhat unaffected by past recessions. Taking Towers as an example, Air/Galactica was built in the midst of a recession following 9/11, and the park still did fairly well from it. Even during the 2008 financial crisis, which produced the worst recession post-WW2, the park still produced very good attendance figures, and Thirteen, which was built when the world was still arguably suffering from the after effects of the financial crisis, produced one of the park's most successful years ever. I'd argue that the park suffered far, far more from the Smiler crash in 2015 than any global recession, and even then, they still produced Wicker Man in 2018. Even if this crisis produces a very bad recession, it shouldn't last too long and it shouldn't impact the parks badly in the long term. The thing with the current crisis is that business should in theory resume with a degree of normality once lockdown measures are lifted, which should see the economy begin to grow again.
Also, in terms of the parks that Merlin owns; they will eventually receive cashflow to invest back into the parks, and I'd personally argue that Alton Towers is one of Merlin's least risky properties to invest into. You may wonder what I mean by this; the reason why I'm saying this is because their major additions usually tend to be very successful (in terms of short-term returns, at least). Thirteen produced one of the park's most successful years ever, The Smiler was a massive success once it opened and Wicker Man arguably reinvigorated the park's popularity and momentum following a hard period post-Smiler crash. Compared to some of their other properties, whose major investments seem to have a more inconsistent success rate (Thorpe Park comes to mind), Alton Towers is a fairly safe park to invest money into. There's also the fact that Merlin is now a private company; an investment's returns don't necessarily have to be short-term now that they don't have public shareholders to rely on, and the company's financial position is arguably far less volatile than that of most other theme park companies out there, as they no longer have a rapidly fluctuating stock price in the stock exchange impacting things.
So in conclusion, I do think that SW9 will get built. While it would not surprise me if it was delayed by a year or two, the plans were revised or both, I do think that we will see it get built at some point in the near future. Park officials have previously hinted that it's coming, and there were suggestions that it was in the planning stages as far back as late 2017. The park may well have ordered the ride and/or signed contracts before coronavirus broke out depending on how far back they started planning it. And going by theme park history; if there's anything that's a safe bet to reinvigorate visitor numbers and get people back in, it's a big new ride.