really, I enjoy sigils I think they are an important part of the game.
The issue is, really, at high-level play I always expect a sigil. This is because they are g'damn useful, which is a GOOD thing. Not using sigils and then complaining about them being OP is a sign of poor play. They are just extremely useful, to the point I always expect them.
This is one issue, the other is that is a very firm winner-keeps-winning item which is probably why so many people are angry about it.
Increasing the cost to 600, make it so you can only carry one at a time really should be enough. Now a person who sigils has to go to base after winning the fight (can't just let monk heal you up) or you won't have a sigil next time, so you will lose for sure, possibly die. Also, it's slightly more expensive so the first sigil comes slightly later in a game, it is a little more of a surprise.
Nerfing its duration really makes it happen like this:
We fight. I sigil to get a kill. You sigil in response 2 seconds later. Because you sigil'd last, you win by default since my sigil WILL wear off before yours will and then I will have 10 hp and you will go LOL DIE.
Nerfing its duration also gives a serious buff to UB. ASSUMING NO MONKS (Sedna counter healing, or whatever), if you have 600 hp before you sigil, you will have 600 hp 10 seconds later. Less if you take damage. Because all that HP disappears after 10 seconds. All of it.
So I hit you a few times, then I spit on you after three seconds. You are now basically dead because your sigil will wear off with 1 hp, Spit finishes you.
I don't think we want that.
Alright, fine, now you get a Sedna heal and a monk heal. Now you'll have 2400 hp 'reserved'. But if you take just 1400 damage in those 10 seconds, then get spit on... yeah you're still dead. Sigils become a ... item of deception. You have a buffer, but the buffer goes away really fast. Everyone will still sigil, it'll just be postponing the inevitable by 10 seconds, where the person who sigil's first always loses.
Nerfing Sigils duration will have a radical upset in a sigil's use, if it is less than 15 seconds. Combined with the upcoming nerf lock games will last several minutes longer as it is, while the losing team will have very little room for a comeback. Do we REALLY want make it even worse?
It's sad how many people want to buff damage and nerf HP stacking but they fail to realize that the more you buff damage and the more you nerf HP stacking the battles become shorter and shorter and more like an FPS. But maybe that's what kids want these days?