Might be worth keeping an old bootable version of MacOS X handy... ...or better still wipe and reformat as Ubuntu, when they inevitably force-upgrade more recent versions.

Remember the latest MacOS will automatically install "security updates only". Soon there won't be the ability to override in the UI (currently hidden) and this will extend beyond minor updates to major ones. What constitutes a security update is changing by the day. Just read the latest legislation out of the UK.