I’m not sure but seems doubtful that you could run a 9 speed chain and crank with an 11s cassette and derailleur, maybe, but seems like the chain width etc would cause a problem.
The more innovation/improvement happens the harder it is to mix and match stuff. Chains get narrower, tolerances closer, and it’s tricky to keep progress going if you’re constrained by trying to grandfather in the users of the last generation of stuff. I agree it would be nice, but I can see why Shimano and SRAM choose to do this. From Shimano perspective at least the 11s and 12s di2 use different wires and connector, so you physically can’t mix them. SRAM imo could have made eTap and AXS compatible much more easily but chose not to, they weren’t constrained by a connector
Two wrongs don’t make a right, but it is what it is. There’s lots of things I want but can’t justify spending the money on even though I can, so I just don’t spend the money
Fun fact, that flattop 12s AXS road chain doesn’t even work with their 12s AXS Eagle group, we’ll maybe it works but not officially