Lifegrip punishes dealing non-lethal magic damage, and maybe too hard

I'm going to start this post off by saying that part of why I'm saying this is certainly out of anger, and here's why:

So I was playing Swain in an ARAM, and I was trying to kill this Twitch with the help of an Ezreal. It was basically a stat-check, but it was one we seemed to be winning. Twitch's items were a , a , and a , as well as a , , and a . I thought we were winning until my multiple DoTs managed to chip him down to 30% (and, well, Ezreal got Blitzcrank hooked), and then all of a sudden not only was the Lifeline shield allowing him to lifesteal back up while I wasn't able to directly hit his health, but Lifegrip gave him more lifesteal to do that with for as long as he stayed in combat. He was close to dead but as soon as Lifegrip went off he was half a second away from half health, because when I tried to kill him I gave him a spike in power.

Maw of Malmortius is supposed to be an answer to burst magic damage for sustained damage oriented AD champions, and it succeeds in this. The shield absorbs burst nicely, and of course burst is supposed to be countered by sustain as evident from recent patch notes concerning Dr. Mundo and the existence of Death's Dance, et cetera. But something that's supposed to help survive burst (or just magic damage as a whole) shouldn't punish dealing magic damage at a certain health threshold, and especially when damage over time effects exist. Maw gives you lifesteal and spell vamp and AD for taking magic damage after which you'd be under 30% health if not for the Lifeline shield, and the Enraged buff doesn't expire until you leave combat. So if you survive any amount of magic damage that would take you under 30% health, you're buffed up for the rest of a fight (or longer if you draw it out on minions or monsters afterwards) if you don't immediately die anyhow, which really makes it a harder counter to sustained magic damage than to burst, because extreme burst like would hypothetically warrant Maw's purchase can just burst through it but if a Swain or a Cassio or some other battlemage is trying to get some damage off they can end up doing more harm than good.

I understand that the example definitely wasn't something that happened only because of Lifegrip, and that it's not been a problematic item, but something just sits wrong with me about an item giving lifesteal for surviving magic damage at a certain health threshold, and the lifesteal persisting until out of combat. It doesn't just help survive damage but it punishes it, and unlike Fiora's W or old Thornmail where the punishment is a stun or a bit of damage for using immobilizing crowd control or a basic attack respectively, giving someone Lifegrip grants them lifesteal and spell vamp and AD for the rest of a fight.