I don't think this is going to amount to anything; the rate he's popping these out suggest he's more motivated by making a point and/or prestige than getting something useful at the end, and turning these enormous codebases nobody has any intuition for into something maintainable is going to be grueling work that will make them wish they started from scratch. But if we imagine that this will go somewhere:

Wouldn't that be bad for open source in the long run?

Businesses, certainly not the larger ones, are not going to touch this for various reasons. So while Adobe and Microsoft may take a hit, it's not going to fundamentally harm their business models. Students, on the other hand, can't afford the real thing. If that leads to them to alternatives like LibreOffice, GIMP, or Krita, then when they end up in the workforce, there's going to be at least some pressure to let them use the tools they're used to. In fact, I use LibreOffice at work. But if they can have something that looks and feels exactly like Adobe's and Microsoft's products, they'll feel right at home with the real thing at work.