Blackbeard and Shanks gain absolutely nothing from their territory and it's weird that you think the BMP can't hold the territory but somehow Kidd and Law can with their fodder fifteen man crews lmao
Blackbeard has a ton to gain. He hunts DFs and also wants their poneglyph. Kid and Law can hold some islands in tottoland, but definitely not all. In any case, Kid and Law both have better scope to grow their crews than Katakuri.
Merely being a strong captain is enough to gain a strong crew. We saw how so many people wanted to join Luffy in sabaody. The same will happen to Kid and Law post wano.
In any case, there's no way katakuri is strong enough to hold all of BM's territories.