I wouldn't assume that either or even both Avatar and Avengers attraction will lack a dedicated wheelchair vehicle, since we don't know the exact experience they may still find a way to include one. Heck even having a drop doesn't preclude one from being added, there are ways to still have a drop and include a dedicated wheelchair vehicle. While I personally don't think a drop is needed or warranted for an attraction like Coco, having a dedicated wheelchair vehicle is not the reason to not include a drop.
Also I don't think having one or more of the new attractions not having a dedicated wheelchair vehicle is a "loss". As long as there are ways to experience the attraction, ie transfer or other means, its still a net positive. As real "loss" would be if they didn't include some provision for guests that need it into the new attraction, which we know Disney will never do now.