Well, the whole 99LP issue is because they have to draw a line somewhere. If they let you go to 100 instead of 99, then what about 98? 97? 96? ... 5? 1? You need to draw a hard line and stick to it. Let's arbitrarily say 95. Then someone is going to complain about 94. That's just how people work.
The 0 LP afterwards is still an increase in ranking. The reason there are promo series is that once you promote you are protected from dropping for at least a few games. Whether you use it or not, that protection is the tradeoff.
Losses result in decreases to LP because if they didn't, you'd just permanently be in promos and could never drop from 100LP, you'd only necessarily either lose every game for eternity and stay at the same rank, or eventually rank up.
So basically my post so far sounds like I'm saying you're wrong. But I'm not. I think the actual problem is this:
If you're at 99LP and win, the game has no memory of that. If you go 1-2 you still end up around 80. If you go 0-2 you still end up around 60.
There should be no cap at 100LP. Your LP should count up. So if you go from 99 to 120, so be it. You enter promos whenever it exceeds 100, but it continues to fluctuate while in promo series. If you go 0-2 you're at 80 now. If you go 1-2 you're at 99 again or 100 and right back in to promos. That'd solve most problems. Sure, you still feel like you 'wasted' a win when you get in to promos from 99LP and win them on that try, but I don't think anyone is really complaining about ranking up (at that point your MMR increase is going to result in you continuing to rank up more easily at least anyways).
