It can be an almost drop in replacement for many projects. You may have to lean on the officially offered https://github.com/developit/preact-compat depending on the project. If you add preact-compact you do lose some of the size advantage over react but that's not really an issue if licensing is your main concern.

Seems the preact author might want to consider marketing preact as not only the lighter weight alternative, but also the MIT licensed alternative to react (Although if more projects included that surprisingly controversial patent clause it would mark the end of software patents. Isn't that a good thing?)