From watching Rich's talks on Datomic, the argument seems to be that for many problem domains, we can spend the cost of immutability to get a much nicer development environment.

If you're already sharding your DBs in a frantic effort to keep your web startup scaling, obviously switching to an immutable database is a bad idea.

But if you can fit every transaction your company has ever done in memory, why wouldn't you go append only?

While the latter might be a niche market today, if Moore's Law keeps up for another few decades it'll be the common case. I'm not clicking on more links than I was 15 years ago, but I've got three orders of magnitude more memory.