I really doubt Gai is in any danger of dying, he isnt central enough for it to have real impact on the story, and the author isnt killing people at random.

As for all the problems regarding the hosts, then to start with all we can say for certain is that people like Kakashi and Gai is in the same class as them, but those people are freaks of nature compared to normal ninja's, and not something you can count on to show up each generation.

With the tailed beasts on the other hand, as long as you find a decent host then you are sure to get 1 superninja, who is able to lay waste to a town on his own.