average CMS developer= average quick cash and narrowing margins as each tom dick and mary starts to do it coz its simple.
Hardcore coder= more money and more sustainable career since demand wont grow much coz everyone is focused on previous point
I think this is only true if your core business is selling CMSes.
If you are in the business of selling, say Pink Land Rovers, - does it matter if your site is Joomla, WordPress, Drupal or a custom system?
They key argument was that start ups were hurting their businesses using readily available CMSes rather than custom engines. If what you are doing is selling cheap concert tickets and music CDs then how do you benefit from having custom everything down to the grid?
If however your core business is all about selling custom CMSes then yes perhaps the argument holds.