So for context, before the update I was a Kog'Maw main. I LOVE the old TF snipe maw gameplay. Mid was my secondary and I used Aquadragon's AP kog build to great success. I also spent a ton of time in season 5 playing Jinx. I'm familiar with you, we've talked on the boards and I really like a lot of the ideas you've had for Kog in the past.
That being said? I can honestly understand why they don't want old AP kog back. Especially with the new Rylai's (toxic AF.) It's not that people can't dodge his ult. It's not that it's even hard to dodge. It's not the range. It's the consistency.
Sure there are other artilery mages, but none of them can sit at 1800 range and consistently deal damage without ever walking into range of the fight. Kog, built full Mana regen and magic pen, used to be able to out-DPS a Xerath without ever being in harms way. At least Xerath has to be in range to use his eye of destruction. And to hit Kog'Maw levels of range? He has to use a high cool-down ult and becomes immobile while using it.
Old Kog had a theoretical higher damage output if he did the whole mid-range magician thing and actually utilized his void ooze, but a lot of the time the range and safety was just plain worth it.
That being said? AD Kog's old ult is the only thing that kept him alive in lane post 6. They will HAVE to find some way to compensate him for that massive of a Nerf. They'll need to do it in a way that affects his lane.
Kog in season 5 used to out DPS jinx vs a team with multiple tanks by a longshot late game if you could keep him alive. The trade-off was the up time of the damage and the reliability of the range.
I'm not really sure how to make him a competitive alternative to Jinx, but that may just be built into the identity of a Glass Cannon like Kog.
I don't think his old ult was Toxic for AD Kog in the slightest.
My last thought is, in order for him to fulfill his full dream of a, "Protect the Kog'Maw," team comp level carry, he should be really difficult to play with an uncoordinated team. If you don't one trick him it should probably be quite hard to pull off. Jinx should never be that. She's a beginner's late game ADC hyper-carry. Kog SHOULD be an ADC positioning stat-check and be punished hard for failing.