At the top level, I believe it is very lucrative. It’s very difficult for any new streamers to establish themselves these days though as the market is so saturated, and it’s hard to compete with the big names with huge bankroll.
Steve has shared his affiliation financials many times on the forum, if you dig around you can get a rough idea around what he makes / used to make. It’s definitely not anywhere near as much as some people think though. It would have dipped significantly while he was on his break too, I’m sure.