The first two lines of the OP are simply factually incorrect.
Sales are not down. They may not be increasing as fast as they were previously, but, they are still increasing by all accounts.
If you took every Spelljammer book published previous to 5e's Spelljammer, added together all their lifetime sales, you might, might, just equal the sales of 5e Spelljammer. IOW, 5e Spelljammer is a thousand times more successful than Spelljammer ever was. So, I'm not sure that you can say they "butchered" Spelljammer. Ravenloft is by far considered one of the best 5e offerings out there. One of the most highly regarded adventures, and quite possibly one of the best selling adventures of all time.
I am constantly baffled by this never ending stream of thought that thinks that 5e WotC D&D is somehow not doing well.

When you have double digit growth, year on year for TEN FREAKING YEARS, at some point we should probably say they are doing things right as far as the hobby goes.
All this, "Well, they butchered this or that" stuff is just people confusing their personal preferences with objective value.