One side effect of supporting .dex as the intermediate form is the entire existing infrastructure (including side-loaded apps, third party app stores, etc, etc)

One side effect of supporting .dex as the intermediate form is the entire existing infrastructure (including side-loaded apps, third party app stores, etc, etc) continues to Just Work(tm). It also allows for devices running different architectures (including those not currently supported) or with workarounds for weird architectural bugs to work without having to coordinate that with a central authority. DEX remains the canonical intermediate and wire form of "native" (meaning compiled-from-java-source) Android apps.