Who cares about source code? Not most users.
If the aim was to make software that's easily user-modifiable - user-modifiable, not developer-modifiable - free software has been a complete failure.
There's certainly a net benefit in distributing software with free access to source code so it can be modified by experienced developers.
But that has never been Stallman's stated aim. If Stallman and the FSF really cared about users - not developers - they'd have spent more time creating software systems that were user-editable.
In fact the industry, including Stallman, has stampeded in the opposite direction.
It's harder for ordinary users to make modifiable software now than it's ever been in the past. We no longer have HyperCard, VB and VBA are dying arts, and the insides of all operating systems - including Linux - are absolutely horrible environments for non-developers to work in. We don't even have BASIC or those make-your-own game systems we had in the 8-bit days.
Those were bare minimum systems. Has it really been impossible to invent something better for more than 30 years?
No one knows. But for all the rhetoric, the question interests Stallman as little as it interests the rest of the industry.