I think it makes more sense to just consider why they're even building it. Their goal is to build an AGI, for which they think they need data and compute. They

I think it makes more sense to just consider why they're even building it. Their goal is to build an AGI, for which they think they need data and compute. They need market reach and revenue to make data access feasible and open investor's wallets for compute, and anything that makes the data easier to get and isn't too hard to do is going to help them on their main goal. Whisper being as widely available as possible is going to result in a lot more human origin language, not just in their services that are trainable, but on the web as a whole. Releasing whisper does basically nothing to increase output of machine generated text, and increases the amount of human text on the internet, so it's a net win. The actual calculation is then going to be on how hard it is to make, and my guess is that for the top AI research team in the world with Microsoft resources, it turned out to be a pretty easy problem to comprehensively solve.