They jumped a few steps on their road to nobility XD, but truth be told... I like what happened here.
Normally in stories similar to this, the MC and co defeat the evil group and for some reason, the remnants of the group implore to become their subordinate, and the MC would just accept it like it fell right in the lap.
But here there is a reason Kakyu decided to take over the group, they already were a pretty extensive organization, with layers of functionality and well organized, with that logic in mind it is less about "falling in their lap" and more that it would be kinda stupid to no make use of those structures.
Sure, he could have kept only a few parts, and delivered the rest to the kingdom's authorities, but why waste an already well-put frame, more so that Erza is just the person to administer the intelligence section of their newly acquired resource.
I hope we get a few more moments in the future for Erza and Kakyu's chemistry, I do think they have it already but time to develop it is never bad.
So... I have a little thought about the future that we know it may happen... I think it would be an interesting idea if Ars really does fight against Kakyu, what I mean by that... I think there is an interesting twist to that happening that instead of it being a tearful situation, it is more about them manipulating fate.
Let's say that ultimately Ars HAS to kill Kakyu, instead of it being an emotionally turbulent situation, Kakyu accepts that, and they plan it out a little to "maximize" the situation, he gives his farewells to the people he loves, and instead of going with pain in his heart and regrets, he does it with his head held high, leaving his family well.
It would still be sad, but I think with a little tweaking there is an interesting development there.
Obviously, there are other ways this could go too, but I think the essence of the idea could make for something more interesting to see.