Yep, MACH isn't just another website and isn't for all scales (a SME would go bust implementing some of these stacks) - as you say the key thing is it's part of organisational change management, not site migrations. The dirty secret about the headless CMS is that it is not a Content Management System - it's a database with some specific Ul and domain language on top for data that can be used as content in many systems - and that needs the plumbing to go with it - and for a lot of businesses those are domain specific.
https://cdn.masto.host/tanecodes/media_attachments/files/112/360/275/380/543/583/original/461baa9c2dda01a7.png