Ok I must be a bit stupid, because personally I'm not really seeing the issue you guys must be seeing. So can someone please explain it to me.
Because the way I see it, yes preview 3 is obviously considerably worse to preview 2 for Missile Spam.
But
Live is: 7% chance to do 10d10 damage + 7% chance to deal 2d100 + 7% chance to deal 1d100.
Throwing a 5 missile spell, on average one of those above will proc. On a full 10 shot chain missile, you'd get on average 2 procs. But a 10~/6~/4~ (or whatever your personal split of missile spammer is, mine is 9/7/4) are only getting 5 or 6 base missiles even in chain missile. With the additional 10 for CM's explosion 2nd effect. So that's about 16 missiles, so on average you're going to proc 3 times. (3.36 times on average if my maths is correct)
Current Lam is: 30% chance to deal 3d100+100 + 30% chance to deal 3d100+180, but only once per cast.
As far as I can tell, base magic missile is still going to come out ahead of live by a large margin. Even chain missile, which as far as I can tell (on average) is the only spell that's going to come out worse, with only getting 2 procs instead of 3 (or heck, I'll be extra generous and say you got the unlikely .36 chance 4th proc), and I'll even be EXTRA generous and say you got 4 procs of 2d100 which is the best case scenario, well you just got 8d100 extra damage. On average, 404 damage.
On Lam, on chain missile, you're getting an almost guaranteed 6d100+280. On average 583. Still more damage than live. Chain missile is, as far as I can tell, on average ahead. And it's the worst case scenario most possible procs. Any other spell that throws less missiles or beams or rays, should be ahead even more.
So as far as I can tell, the only people who would be disappointed by this, are people who tested it in Preview 2. Anyone not watching Lam, who's just playing the game normally when U51 hits, would just see a buff. Which means if they're viable on live right now (Which, they exist, so I assume that means they are.) then they should still be viable after this update hits. Just less powerful than they could have been. And to be honest, i tested it on preview 2. It was silly. I liked it but it was silly.
Note: This is assuming Prism and Stay (F/G) both have a different 0.1s cooldown. If it's a cooldown they both share, that limits procs to 1 per cast fullstop, then yea that'd be bad. But I would assume it's not that.