If my kid told me they wanted to be a twitch streamer I would advise them against investing a significant amount of time and effort building a business with a single gatekeeper.
If they wanted to be a famous personality, I would insist they start building a profile on every platform.
This isn't well known, but to monetize on Twitch (i.e. be able to receive subscriptions and bits), you have to sign an affiliate agreement[1], which includes a clause prohibiting you from multi-streaming, or putting your VODs up anywhere else for a full day after their conclusion. This severely limits your ability to cross platforms.
One is taken for granted, but I think it is a valid point. I share your opinion (bad to depend on platforms), but that never might have triggered the comment.
For some people never means usually not, and for some never means never :)
I'd advise them to dominate a new platform as an early adopter and then spread out from there. Or put out content very consistently on 2-3 platforms. But even spreading yourself between two accounts let alone multiple platforms is time consuming.
If they wanted to be a famous personality, I would insist they start building a profile on every platform.