But enabling/disabling Stage Manager is a two step process (swipe down on Control Center and tapping the icon).This would be easily solved with a pop up window that asks if you want to stay in current mode. Apple’s good at this.
I don’t see a huge need for a feature that will automatically enable Stage Manager when docking a keyboard to it (imo).
Tbh, that’s entirely up to Apple. If I’m not mistaken, they control who has access to the Smart Connector… I‘m sure manufactures would want to create what you mention.Those stands look interesting. But a missed opportunity that they didn't add power though a smart connector, like the Magic Keyboards can do.