yes they are, and if there is going to be a kh3 (which the producers said there will be) it would not make sense.
just look at kh2, no one knew about namine, only that she was a special nobody of kairi. and organization 13 popped out of nowhere in kh2. the only way u could understand kh2 completely was through chain of memories.
if u are a true fan, have an open mind to the new games.
there not spin offs, since they are still essential to the series.