I think you're underrating Spellcasting as Net hits also increase damage. Magic 6, Spellcasting 5, Spec, and Expertise, Rating 4 Foci, 13 dice at chargen, up to 18 pretty quick. This is in addition to the brute squad of nigh invincible Spirits doing whatever they want. Plus spells have the nifty option of being Mana or Stun damage, which in many situations helps with the security/game world response.
Oh, don't get me wrong, magic wins. Always and period. The only aspect I am examining is aoe damage, both stand alone and in comparison to other aoe.
Yeah, net hits will totally factor. But my opinion that grenades are unacceptably way too strong stands.
Grenade DV is too high? Lower them to 4 DV its still just a matter of more grenades. Either bust out the Semi Auto Launcher and fire off 4 per turn per Gun Bunny, or have the Rigger's Drone swarm saturate the area. I guess more PCs get to fire off grenades than just whoever wins initiative?
4 DV is infinitely more survivable than 16, 12, or 8 DV. Right now, a troll with 14 soak might survive 2 ground zero grenades with lucky rolls with luck. Most characters with average body (3) cannot mathematically survive a single grenade landing within a meter of them.
Give Grenades a defense test? Sure. But an optimized Samurai or Physad or Rigger will rarely miss with a gun, unless you've got a significantly different mechanic in mind, I don't think it would be much of a change. Could be wrong. Defense tests tend to favor PCs, not NPCs. Most stock NPCs don't dodge real well, and I think that may be deliberate.
First off, under no circumstances ever should any effect that affects another character not require a directly opposed roll. That is poor game balance imo.
Second off, it very much depends on the character and action economy in question. A perfectly optimized elf samurai will hit a decker the vast majority of the time, but will never have a snowball's chance in hell of hitting a mystic adept with increased intuition and reaction, max ranks of combat sense (adept), and using the dodge action. Even a fairly non-optimized character will have their fair chance to dodge several grenades in a bombardment situation if they take full defense and/or dodge knowing they will need it t
Grenades haven't changed that much between editions. There was a Burst Fire launcher in the 5e CRB I think and Chunky Salsa was a thing. If a 'Runner team wanted to blow the hell out of a run with Grenade launchers in 5E, they could. Most teams chose not to because they didn't want the heat/consequences/notoriety/whatever that comes with decisions like that. That calculus should still be valid in 6E. Clearly YMMV.
Again, it depends. Chunky salsa, while possibly very realistic (I have zero experience with explosives), was a really terrible game mechanic. Second, except on the most extreme ends, it was possible to build characters that could survive it, and even laugh off the damage entirely, due to the substantial difference in soak pool potential. One of my main vets in Missions was a Minotaur bio-sam named Cocaine Bull, and he had something like 71 soak dice + 11 auto hits from hardened milspec? Dice pool might be a pinch off, but he routinely entirely shrugged off hits that characters have no earthly business surviving.
Grenades suck as they are for sure, but they have always sucked but even more so now unless changed which is on the wish list so we shall see what comes of it.
As for a little bit of a side rant this is the type of thing that makes me hate Missions play because I can't control what the players do at that level but in my home game grenades are a no go for everyone.
Of all the stuff I saw come up that was occasionally problematic, grenades rarely did for me personally, other than from Tony :p. The majority of actual problems I ran into was the rare player (not character) who was a pain. That and god mages, that is mages who just acquired too much karma and became deities.
That is an understandable frustration though. Some things are better off banned for all sometimes. I always thought grenades availability was entirely too easy.
I'm glad you posted this, Lormyr. At the risk of stepping into a duel in progress between you and Hobbes, I did want to talk about grenades! :-)
My GM and I laughed and laughed at that. 
Thanks man, and the conversation is welcome to all. And I stand by that quote too!
1) The only time someone gets hit with the Ground Zero damage is if they are literally standing on the grenade or its in their pocket. Otherwise, they get the benefit of the doubt as close range.
2) The Avoid Incoming minor action is not affected by dodge penalties; 1 hit equals 1 meter moved.
3) Both the Avoid Incoming and Hit the Dirt minor actions have the following extra bit: "This action gives you X auto hits on a damage resistance test against blast effects (e.g. grenades)." I'm thinking X is 2, maybe 3.
That would help to take some of the bite off, but I personally believe it still leaves them too strong. Lower damage and give a defense test, but if for some reason we won't do that, having the avoid incoming action just flat reduce damage by 1 per hit would also at least help.
Eh. Maybe, but I expect it to have enough teeth to deter the kind of players who use those tactics at cons.
That's the real issue. A decent player who has a problematic character can just be reasoned with on the player side and scale back. A problem player with a problem character I personally would just eject if attempts to reason with them failed.
But that said, is grenade spam a problem? I am torn. On the one hand, I can see how it could be. On the other hand, hard to blame players for using tools the game equips them with. Best solution remains to take the teeth off of them so they remain a good option without being perfect annihilation.