Ah, the classic "everybody brings sentries" argument. There's an 8 year old argument about whether TA beats QoP in mid or not that revolves around this very thing. I think the general consensus is going to be that it's a lot easier for a 500 range hero to deward than a 100 range hero, which means bringing counter wards as TA is generally not intelligent unless you think they won't keep buying more for some reason. Anyway, sentries remove meld from the game, allowing a hero like Viper to beat TA out of her refraction and kill. Assuming the Viper doesn't go afk and eat psi blades spill all the time, it should not be a very hard lane for him.
As far as finding low mmr players goes, I think the effect you're seeing is that weak heroes do poorly in any game now, and lower rated players tend to care less what hero they pick (I'm a prime example of this). That's because picking broken heroes is the best way to gain MMR, so the people who aren't Leshrac in your game are bound to be lower MMR, as they aren't taking advantage of (see: abusing) the patch. Here's an example
http://www.dotabuff.com/matches/1792482333
Guess who the worst player was? It wasn't ranked, actually, so no way to be sure, but I'm going to say 99% chance it was Sniper. Is it obvious because of the way he died to brood because he ran around in the jungle under webs with 200 health like an idiot, or is it obvious because he actually picked Sniper in a game like this? Or in any game? I know 4ks go and die a lot for no reason. 5ks do it, too. 6ks don't exist, as noted, there are 140 of them in the western world so probably not more than a couple dozen on at any time.
You're also a mid player, which removes that entire lane from the equation when it comes to guessing who the awful is. I frequently have the lowest MMR on the team as my mid, and it's less obvious when he's still 10000 gold ahead of anybody else simply because he is mid and Leshrac.
Playing mid is almost certainly the only way to reliably gain MMR in this community. Having a mid that does well goes a long way to preventing the rage throwing, I'm sure. I've really been trying to find a reason for it, because it happens to me constantly, and I would say is actually a majority of my losses, and yet it doesn't seem to happen to other people hardly at all. I don't see how going mid prevents your support from getting pissed off at your carry and going afk, but there must be some magic to it, as the people who don't complain about throwers tend to be mid players. I've accepted that winning solo ranked has nothing to do with skill, at this point, it's all about being able to stop your team from giving up 5 minutes in, that's how you win.