I think this question is more relevant now than when I first posted it.
I'll bite, and try to answer the original question.
I think if I'm WotC, I'd be trying to analyze the data around WHY. My hunch at this point in time is that it would have little to do with a system change (as I don't think the mass consumer market cares if the book labeled D&D is 5e or revised OneDND 5e - I don't believe there is huge understanding of the playtest and update within the broader market), and more to do with the state of the market.
Obviously, the pandemic was a huge boom for D&D, so I would be looking at the impact of that, and where it pushed growth.
I'd be looking at partnerships and how they are positioning the product, like Critical Role.
I'd be inspecting every aspect of the marketing of the product and seeing if there are new or revised ways that can impact the sale (should we do more Twitch streams? Join D&D Tiktok? Work more closely with Youtube personalities to hype product through actual play, interviews, special events, etc?).
Is local brick and mortar way out or under performing, and if so how do we capitalize or improve there. Can a bigger push into big box retail add significantly more sales?
I'd be checking in directly with Amazon to see what could be done to boost sales on that platform. Is there advertising, partnerships, or SEO stuff that could be done to improve sales directly with the platform? Does Amazon want to produce some DND content for Prime? Maybe for the Kindle platform? Maybe for their gaming division? Maybe with Twitch? Amazon controls a lot of platforms that could synergize for some major brand and product marketing, so this might actually be a major focus.
I'd be looking at VTT options, and seeing if there are ways to push product in the digital space beyond DND Beyond.
I would be pushing hard to get our own VTT online, to try to capture market and bolster the company that way.
I'd be looking at partnership deals. If the movie or television show is successful, push more of that to synergize with selling content. I'd try to find other publishers like Larian to make more quality video games ala Baldur's Gate 3 (maybe work with Owlcat?).
What I wouldn't do is just walk away from One DND. At least not unless there was very specific and direct data that showed the changes were the major cause in drop in sales.