I'm a diamond malzahar main, and I've mained him for four years now (diamond last year and this year). I think I've got a good background for talking about what malzahar can or can't do.
In mid lane, the other champs will basically walk away from you and the voidlings cannot chase, because they slow down out of auto range. With the futher nerfs, anyone with aoe (so, virtually every mid laner) kills your voidlings before they can even auto, unless you blow your entire combo: ie the other player either needs to make mistakes (not uncommon in lower elo, much less so in high level) or you need to all in the other player just to do damage, beyond the paltry 250 you get from e at level 9, BEFORE resists. In practice, it's more like 140-160.
"But q resets e!" you say. Yes, until you realize it takes a quarter of your mana pool at level 9 to do it, even after buying a mana item. That is to say, you get punished for harassing, and it does minimal damage when you do, making it functionally worthless unless you use both charges of w (see above for why that's not useful unless you all in). If you DON'T all in, that's a grand total of (assuming optimal timing on the q) 300-ish damage from e and 80 (50 after resists) from q. (I'd also like to challenge you to name one other mid laner whose harass does all of 400 damage at level 9, assuming everything lands. I'm not talking about one ori q or something; if it's a thunderlords champ, then eg ori gets off q+w+auto+thunderlords proc. That's 500-600 damage right there, and it's on a much lower cooldown and much lower mana cost.)
If you do use e+q for harass, then any mid laner with waveclear (so, once again, virtually every meta midlaner) will use one or two abilities and shove you under tower, then roam and get a kill from another lane, while you're stuck autoing minions for 60 damage a pop. If you don't use e+q for harass, and use it for waveclear, it still takes so long that even a mordekaiser can just walk up and clear the wave faster than you (not making this up, it happens from time to time when I play against morde in mid lane. You can't even punish him because your voidilngs die even faster inside the enemy minion wave when you have no minis and his shield just powers through your e and e+q).
Once the other mid laner gets qss, even as a first item, your kill potential drops like a rock, along with the fact that the mr directly reduces your damage anyway, since you can no longer all-in and (as previously established) not going all in just gives you the garbage damage from q and e.
He wasn't unplayable before this patch, but he's pretty much dumpster tier right now. It's a little easier once you start teamfighting, and have the items to make q actually useful, but his voidlings still die instantly and you still get either cced or qssed when you ult, basically turning him into a q bot.
And riot claims this is healthier than old malzahar, and keeps nerfing him instead of a) reverting w change b) giving him scaling at the expense of base damage, the easiest fix in the book for nerfing supports without hitting the other roles and one they do to literally every carry that builds tank items and is successful or c) reverting the rework entirely, which is what almost every pre-rework malzahar player wants anyway.
So that's my relatively well-qualified view on this matter, and I don't understand why riot is so against admitting that they made a mistake.