Yes, we considered it. Two arguments against this:

1. A single “async component” is useless because the power of async rendering is in letting React coordinate the work of the whole tree. As long as just one leaf isn’t async-compatible, the whole tree isn’t.

2. In practice most components already are async-compatible. So we don’t want to hold off progress by forcing people to explicitly specify it. Instead the plan is to outlaw patterns that we know won’t work.

Hope this makes sense.