I know corporate America doesn't really deserve any meaningful amount of good faith, but for whatever truth is worth, "sustainable" in a business context has essentially always meant financials. A platform like Twitch is generally going to have really high operational costs between infrastructure, network traffic, engineers, and revenue sharing with streamers, and given that Amazon doesn't operate Twitch for charity any more than you do your job for free, they need to make sure that they actually have sufficient revenue to be able to make the finances sustainable. I won't pretend to know how profitable it is, if it even is yet, but cutting employees is obviously a pretty easy lever to pull to reduce costs if your operations can get away with it.