As a tangential comment, the plan is to use persistent containers per project for continuous development locally (hidden behind a feature flag today SNAPCRAFT_CONTAINER_BUILD). Furthermore, specifically on build.snapcraft.io, support for build-snap as an entry is forcing a move of launchpad to build in lxd containers.
Now that that is out of the way. Considering we are building in containers we can avoid user interaction for building with --allow-classic transparently in the builders and regular users wouldn’t be surprised. I’d like to leave interface connection discussions for a phase 2 part of the implementation as 90% of the use cases for build snaps are based out of using snaps that are classic confined.