the thing is that while ddnite is not so hot right now (cough my signature), it is far from the only thing dragonite is capable of doing. it is much more versatile than volcarona by virtue of a more flexible movepool and typing.
there are admittedly a lot of pokemon in this game that tend to put big appreciation on spin support, dnite is one, but not all its sets require the spin, because not all sets need scales on right away to ensure they can't be checked by every ice attack ever. bulkier supportive sets (parashuffler, subroostDD) are quite independent of spin support.
ultimately while dnite is demanding of spin support as well, it is nowhere near as demanding as volcarona, and it is also a lot more versatile. i don't see them being comparable enough to drop dnite to B
there are admittedly a lot of pokemon in this game that tend to put big appreciation on spin support, dnite is one, but not all its sets require the spin, because not all sets need scales on right away to ensure they can't be checked by every ice attack ever. bulkier supportive sets (parashuffler, subroostDD) are quite independent of spin support.
ultimately while dnite is demanding of spin support as well, it is nowhere near as demanding as volcarona, and it is also a lot more versatile. i don't see them being comparable enough to drop dnite to B