Earlier quoted context omitted.
You answered your own question. Twitch is solid and well-built. It doesn't need an army of well-paid engineers to maintain it and isn't seeing a great ROI on new features. Some companies repurpose all, many or some engineers when the spreadsheet says to, others fire them wholesale.
Not many people realise this. Engineers build stuff. When your product is feature complete, you can get rid of most of them. Big tech companies keeping tens of thousands of engineers on staff despite their core product not seeing substantial changes for years are just wasting money.
This is all fine and expected, you sacrifice engineering quality for development speed in the early days. Just expect to pay it back by keeping a large engineering team for many years to come, or see your entire tech explode in flames.
Those 'years' are finite however, Twitch has been mature for like a decade already. So finally the tech debt is all paid off, and given failure to develop major successful features, time to cut the staff.