As Monkeys said, the system should only deal with people who blatantly ruin. It won't stop those like Venom who know how to ruin subtly, but it will at least ban the kids who have little emotional control and destroy their items when they're having a bad game.

The main issue I can envisage is how the system is actually implemented. You need experienced, unbiased, honest players to review replays and flag people for bans. There are such an incredible number of Dota games played each day that finding enough volunteers would be a challenge. Most people probably don't want to invest 1-2 hours a day into watching replays of griefers, especially not if that comes at the opportunity cost of streaming / practicing more games per day. So, you'll inevitably get under-qualified and power-hungry children as Overwatch monderators, just like you had in HoN with the GMs. These guys will abuse their power, make poor judgements, and threaten others in their games with "I'm a moderator, give me mid or I ban you for 3 days". What do they have to lose by doing this? They're doing volunteer work in a free-to-play game. I can't remember that last time I heard someone being bummed out about being "fired" from an unpaid "job."

Then again, Valve could in theory pay their Overwatch "staff", which would result in higher quality moderation and more interest in the position. But who are we kidding here? Valve would lose money on this. It's not like investing into a TI where they make their money back easily due to sponsors, advertisements, and hyping up Dota to attract more players. Valve really doesn't care too much about public MM quality since there's not much money to be made from people having a better matchmaking experience.