I would be in favor of having extConf behave the same as the rest of the configuration, regarding the PID thing, but possible with a zero value for global (which could be the default, and therefore be straight forward for any extension, as anybody can define the basic configuration for the whole website) and being able to override extConf¨¨ on a per page manner (I’ve encountered the use case in the past, where I wanted to have indexed_search configured in different ways accross multiple websites in the same system but that is not possible at the moment without some clunky workarounds).
To make it short: let them override what they want.
To make it long: You are absolutely right, this might be a problem, if someone decides to override something crucial which they shouldn’t on runtime, but if the question cannot be solved on how and if configurations should be overridable, I’d suggest that for the sake of improvement, everything could be overridable in a first instance and made immutable in v13 or v14, when the understanding on how the community uses the new configuration system is available. This way, the understanding of what really needs to be overridable at runtime (although it seems pretty clear that most constants should be, at least) and what absolutely should not (database credentials?) and what maybe shouldn’t be overridden (thinking about indexed_search configuration where most stuff really shouldn’t be overriden but other stuff really should be overridable) might be hard to grasp from an extension/core developer’s point of view, since they are most likely not the people implementing the solution for real life problems.
I’m on your side, regarding unifying the configuration and I’m curious to see how this turns out.
Thanks for the good work!