[blueslurpee]

The technique that you have described on viewing the simple object combinations as "projections" is an excellent one; composability in your system emerges from efficiently selecting and combining these projections into the desired combination. At different points in your system you take just take different projections. Cross-cutting concerns are a breeze.

It actually all starts to feel like.... SQL! State is stored globally in a defined schema and queried as needed by the system.

But you can't do this if your compositions are preordained from on high by a rigid class hierarchy, the data is crystallized into the "blessed" projection and that's that. It's analogous to your SQL queries being constrained to solely static views. No JOIN. No GROUP BY. No WHERE.