Charge for streaming
Instead of monetizing via ad's charge the streamer for the ability to stream.
At the moment there is no risk involved in streaming for the streamer which just produces a lot of low-quality streams without effort from the streamer. These people have nothing to lose.
Instead, charge EVERY streamer 1 US cent per minute to stream ($0.60 USD per hour). Streamers have a "Twitch Wallet" which they can credit with Credit Card or Paypal. The cost of a stream is automatically debited from their wallet.
Give every streamer the choice to run ads or accept subscriptions/donations. Streamers keep 100% of their ad revenue and subscriptions and they are paid directly into their Twitch Wallet.
Streamers can control their income and take out money from their Twitch Wallet back to their chosen payment method. Streamers can then see if they are a commercial success or wish to simply be a hobby streamer and pay for their hobby like any other hobbist.