Instead of basing your response on speculation, why don't you play AP Kog'Maw yourself :)
It's annoying having to explain the strengths of a champion to somebody who has never played said champion, rather it's pretty much pointless to someone who hasn't touched the game since season 3 but here goes:
the scenario where it's strong doesn't occur often enough for it to make him viable.
The scenario of enemies being at 50% health or lower occurs easily if you are able to dish out your Q + E damage or your teammates do damage. Honestly the only time when this situation wouldn't occur is so rare that you would literally have to be afk in fountain for it to never occur.
I'd much rather have reliably good damage than an execute and I'm sure my team would prefer it too.
AP Kog'Maw does have reliably good damage as well as an execute. Nobody is forcing you to play it if you are not confident in your ability to carry :)
It also means that your laning is even more garbage because you're not going to be facing someone that's below 50% HP normally.
Please tell me why you think it is necessary for your opponent to be below 50% health in lane when your goal as AP Kog'Maw is to scale mid-late game with a few items and then being able to deal fatal damage?
Kog'Maw's laning is actually scary good. It's on par with Anivia. E + R is good enough to kill an entire minion wave at level 6.
Also, keep in mind that his ult.'s mana reset and mana costs were heavily nerfed
"heavily nerfed" Last I checked, increasing mana costs by 10 per cast didn't count as being heavily nerfed. And the added 4 seconds for his ult cost reset? No biggie. Everything that was recently changed was an expected power tune.
It's not even just the change from a poke tool to an execute.
AP Kog'Maw still benefits from the long range harass from his ult. You say it like you should never use his ult to poke. Resource management, ever heard of it?
Almost every time someone's below 25% HP, a normal ult. would be just as useful because someone else on your team will likely flash for the kill or throw out some spell. In a sense, they are "overkilled".
Oh yeah? Like when? What ults? You have clearly never experienced an AP Kog'Maw with
if you are comparing his ult to other ults as an execute. But it is something you need to see in action in order to understand its power. Lmk if you know of a champion with a spammable ulti as potent as AP Kog'Maw's ok buddy?