I'm not convinced that lowering the barrier to entry to software changes will result in this kind of change of norms. The reasons for closed-source commercial software not supporting customisation largely remain the same. Here are the ones that spring to mind:

• Increased upfront software complexity

• Increased maintenance burden (to not break officially supported plugins/customizations)

• Increased support burden

• Possible security/regulatory/liability issues

• The company may want to deliberately block functionality that users want (e.g. data migration, integration with competing services, or removing ads and content recommendations)

> That was a genuine blocker for me because it prevented me from answering my question.

It's always been this way. From the user's point of view there has always been value in having access to the source, especially under the terms of a proper Free and Open Source licence.