One problem is that it's hard to see how decentralizing work for profit could ever be more efficient than centralizing it. In the decentralizing case you must do the additional work of dividing the task into separate pieces, distributing the task, and validating that your workers are actually producing valid results and not trying to cheat you somehow. In the centralized case, you don't have any of those problems.

Analogously, if I wanted to have a "digital money" I could just have a key value store database running on my home desktop. I could probably handle multiple thousands of transactions per second - thousands of times better throughput than bitcoin, and with only a single machine. The centralized case is just much simpler, faster, and more efficient than decentralizing. I think the sample concept would apply to useful work.

Fully agreed, but... We are not looking for "digital money" here, that is being done by China, the EU and others, and as you're saying, a centralized database does the trick way better than Bitcoin or any POUW scheme.

We're looking for "fully decentralized, immutable and trustless digital money" - and it would be nice if we could combine that with useful computation instead of burning electricity.

Also consider that there are many computing tasks that could be enormously useful to the human society, but not profitable, with very distant rewards (usually investors prefer short-term profits over long-term societal gains), or very risky.