The point is that it is not really a vaccine. A vaccine is against a specific threat, like this or that virus, and if it is good, against this or that virus class.

This stuff works by keeping your immune system on a hightened alert status, so it can react faster to any incoming, known threat. The known part is important here.

And if this "keeping the immune system on a permanent alert status" was any good on the long run, nature would long have normalized this, so there must be a severe drawback to it.