That would be nice in an ideal world, but one of the reasons most packs are small or spread out over a large area like Sebastian’s is because so many shifters in one place would attract too much attention from humans. If 5000 "people" suddenly inundated a relatively small resort in Montana, somebody's going to notice.
Also if all the small packs sent their most able bodied fighters, and they were killed it would leave that pack with little protection. (Say a pack had 25 members and 5 or 6 go to help and then die, they lose 20% of their pack... devastating)
Logistically, it would be too hard to coordinate the influx of so many shifters, not to mention Sebastian would have to thoroughly vet everyone which would take too long. Then there is language differences too.
Nice idea though.