I think (1) is really about where you land it. I am not free to name the product, but I worked on a product that has done billions of dollars in revenue that d

I think (1) is really about where you land it. I am not free to name the product, but I worked on a product that has done billions of dollars in revenue that designed a bunch of state management checkpointing using a custom STM implementation for C (yes, C). It made so, so many things straightforward in terms of corner cases; we also extended transactionality to sync over the wire in some cases. I also think STM-adjacent designs are gaining some traction - for example, FoundationDB looks an awful lot like an STM system from the point of view of applications, much more than it looks like a traditional database.