> Why would anyone think the same thing won't apply here?

Because there's nothing to "move up into" other than "spec, optionally design".

Because the jobs that had "spec, design but don't code" already existed for decades, and pay less than half what the "design + code" person earned.

Not sure what you mean by spec and design, but around me, that's always been paid more than simply coding. If you have a clean technical spec that's detailed enough, the code naturally flows and is often left to more junior engineers, with more senior folks reviewing the code but rarely writing it.

They only said the jobs won't go away. They didn't say it would pay the same :)