My intuitions about KANs and visual data comes from an impression that it would be hard for a decision boundary on visual data to behave nicely if it could only be built from b-splines.
Judging the usefulness of a machine learning architecture is not a matter of determining which architecture will perform the best in all scenarios.
But MLPs are not good for everything. Where Simulated annealing works better than auto-diff is the classic example that is easier to visualize, at least for me.
Even if the sequence 'exists', finding it is the problem, it doesn't matter if a method can represent an unfindable sequence.
That said, IMHO, MLP vs KAN is probably safer to think of as horses for courses, they are better at different things.
At least with your definition of 'usable' being undefined.