I think there are two levels publishers have to be concerned about 2014 vs 2024 5E rules. There is the reality level, how different are the rulesets from each other really? And then there is the perception level of reality. How are consumers looking at the difference, and how do I as a publisher help clear up confusion rather than add to it?
If I design a product using the 2014 rules . . . it's perfectly compatible with 2024 D&D products. If I design a product using 2024 rules, it's MOSTLY compatible with the older 2014 products, I'd want to add a sidebar on using subclasses and species with the older rules. But overall . . . yeah, it's all compatible!
But, regardless of the reality . . . if my consumers perceive a major difference between the 2014 and 2024 rules, than can affect my sales if I lean one way or the other. So what do I do? How do I design my next product, and how do I address the two different rulesets?
During the 3E era, I remember being surprised at how many folks in the community, both fans and publishers, treated 3.5 almost like an entirely new game. To me, the differences were not significant and I wasn't bothered by the "3.5 update" at all (as a fan, I'm not a publisher). But there was the perception the two versions were incompatible enough to invalidate earlier purchases, and this had an impact on the publishing community.
It's why WotC has tried to muffle language making the two 5E rulesets as significantly different. They are not calling the new rules "5.5" or "revised" or anything really . . . which I think was a mistake, as it obviously punts the ball to the community, and we are all over the place with how we are viewing the new rules and what we are calling them.