Yes, but the twitch-numbers reflect a streamers potential for earning money through merchandise, promotion, sponsors and other revenue generating activities. There usually is a direct enough link between them. Tipping is a bit more special, but it's quite unlikely that a small 20 viewer-streamer will get a million-dollar-tip regularly. So you can make an educated guess of the general income, at least regarding someone's success as a streamer.
Of course it's always possible that someone is far more successful outside of twitch. Like an established celeb who streams without monetization. But I don't think it makes sense to discuss those special cases here.
Yes, the viewer numbers are the direct link. But viewer numbers are not Twitch revenue. Subscriber are Twitch revenue. And it is not at all impossible to have a lot of viewers and not a lot of subscribers.
How are you going to know what the stream is earning the musician keeping contact with his fan base?
Of course it's always possible that someone is far more successful outside of twitch. Like an established celeb who streams without monetization. But I don't think it makes sense to discuss those special cases here.