1920 x 1080p have 6Mbits of bandwidth, limiting this for viewers that not contribute to 640 x 360p like max quality will reduce money cost, so with the saving, twitch, can pay much more money to streamers and get a business much more profitable. So the clients would are happy of see that some more of their contribution go to favorite streamer. Twitch should explain the new change for comprehension of clients. The lacking viewers will one day contribute :).
You can also give 1 month of better quality to those who collected a certain amount of bits

calculations as an example:
360p at 2Mbits
(1/3 of 1080p)

Saving
4Mbits of 6Mbits
(2/3 of 1080p)

3Mbits(average) * 3600 (seconds)
= 10800Mbits (1 hour of stream)

(((10800Mbits * 4 hours) * 30 days ) * 5 Millions viewers that not contribute) * 1 streamer

6,48 * (10 ^12)Mbits at month

This month * 500.000 streamers

3,24 * (10^18)Mbits

turned into money(1000Mbits at U$D 0.10)

((3,24 * (10^18) / 1000Mbits) * 0,10 cents)

U$D 3,24 * (10^14) --- U$D 32 400 000 000 000 000
XD