Revert slow decay on Karthus Wall?

Back in season 5 (patch 5.24), Karthus' W went from slowing to the described amount in the tooltip over the duration to 20 / 25 / 30 / 35 / 40% after 1 second. This was fine, because Rylai's was a much stronger and more commonly-bought item on Karthus.

In the current season, the W doesn't seem to perform well (at least while it isn't maxed, which is most of the game). In many cases, casting it at maximum range won't allow you to land even a single Q on the slowed target.

Ironic that one of his lines when casting W is "You cannot escape" when all they really have to do is keep walking away.

I propose that either the slow decay in a more linear fashion, decay less, or have the W just cost less mana at lower ranks. 100 mana at level 4 is just about 20% of your base mana, and that mana is too precious a resource in the early game to be using it on abilities that aren't very effective.