This sounds fine to me, continuing the typealias Default... trend we have already 
This is also likely to influence how we'd set the global ExecutorFactory as well. We leaned into the SwiftSettings earlier because the default isolation was doing that, so we might plop back to the typealias.
Currently the main and global executors pitch doesn't really say we'd do the settings route, and only documents the command line option and SwiftPM setting.
So follow up discussion here might be if typealias DefaultExecutorFactory = is a thing or not, and it's just the compile time flag / swiftpm setting and if so, how it'd interact if a command line option was passed as well? It might be that we don't want the typealias for the DefaultExecutorFactory but it'd feel a bit surprising perhaps, idk maybe @al45tair had thought about this already some more? We did consider the typealias a while back, but moved over to settings influenced by the other proposals in flight AFAIR.
The executor factory is global though, and not per file, so perhaps the difference here would be reasonable. We did have discussion about "only allow it in the module that has the @main" or something similar though.