A "failed new edition" is a bizarre and frankly slightly paranoid idea. The closest D&D has had to that was 4E, and that took vast effort to create a "failed new edition". The stars had to absolutely align in a totally fantastic way.
To be successful with a new edition, all they really need to do is two things:
1) Retain 2E-ish levels of backwards-compatibility with 5E adventures.
2) Implement a relatively cautious 6E that's more like 1E-2E than any other edition change, implementing all the ideas they're half-implementing in 5E, and fixing some stuff.
Combine that with attractive new art and visual design, maybe better writing, better optional rules and DM suggestions in the DMG, and later an updated MM (one of the selling points will be that you don't need updated MMs initially), and boom you just sold a ridiculous number of physical and digital copies. If you release you own digital product instead of letting DDB take your money, at the same time (or just buy DDB), you make even more money.
Re: "even more cautious" and "trialled in another format", I think this is half-true. They're likely to stick to a 1E-2E-style incremental change model, rather than the revolutionary changed of 3E/4E/5E, so that is "more cautious" than previous editions. However, I don't think they'll but putting out 6E's changes in another format, because it would be incredibly obvious and worse PR than just putting out 6E. I do think we may eventually see another RPG with a different system from them, because of the vast potential for selling it the now-vast base of D&D players, but not for a while yet.