At least its plugins, which are what needs to be regularly updated as platforms change, are open source. I imagine someone will clean-room reverse-engineer the core app and make an open source one that uses the plugins.

Reverse-Engineering and reimplementing something is a lot of work, especially if it is a moving target that is still actively developed. I don’t expect anyone to do it.

Also, Louis Rossmann said they only reserve the right to go after forks to prevent a situation like all the shady ad-ridden NewPipe ones flooding Google Play Store.

That is some kind of hand wavy reasoning that might come from someone that could be sponsored by them. Louis should do better than taking any company by their word and promises. And spreading FUD about NewPipe (and by extension all of Open Source software) is also a straw man argument. An untrustworthy software repository is not a argument against the open source software application, but against the software repository and their maintainers.

If you are concerned with that, you should stop using Google Play store.