Ryuma grave was robbed. Also, kappa did take all the swords from grave to be used in war.
So there is a chance that Zoro carries Shusui in future as well.
But I get your point
Kawamatsu primarily took the swords to prevent them from being stolen by grave robber.
The secondary reason was to use the swords one last timeto defeat Kaido out of respectfor the people that died causethat would have been their wish.
So even if Zoro carrier shusui, it wont be permanent.
Personally after we were told black blades are made by haki, it sealed the deal with shusui being returned. Zoro has to make swords black himself otherwise it is pointless. He wont be able to match the likes of Ryuma and Mihawk if he relies on a sword that was made black by someone else