They tried earlier to phase out waterwalking entirely outside of the expansions the mounts were earned in.
They can and do remove many expansion-linked items at the end of the expansion.
Turning waterwalking from an inherent property of particular mount into a function of an expansion-linked item means it can easily be disabled at any time in the future.
They tried to remove waterwalking in current content before. Now it will be easier to do. You’re here to try to distract people from that inconvenient fact.