I think at this point we might as well hold off and see what all is coming with Runes Reforged before we suggest any item alterations. One reason Rylais is seemingly ineffective is the abundance of movespeed enhancements, like Stormraider's Surge. Once Masteries are gone and we can see what similar Keystones, if any, exist next season, then we can be safe in tuning items.
As a Malzahar main, I still build the item almost every time. Especially as a rush before enemies can hone in on their movement advantages, Rylias' 20% reduction can still be pretty strong. The Voidlings losing their bonus speed was painful, but it's more in line now with how Malz used to initially play. Voidlings didn't receive movement bonuses prior to the Mid Season Magic update. I will admit it's frustrating at times for champs with dashes or built-in speed buffs to just walk away from a perma-slow on your E like it's nothing, but other champs are still devastated by the slow (even if the number of said champs is decreasing these days lol). Right now, if Rylais were any more useful, it'd be a must-buy again, and the battle of the slows meta is really annoying in my opinion.