We should also consider that Canon may believe the 'problems' are minor and/or irrelevant.
Anecdotally, the use of EF lenses on APS-C bodies prior to a FF upgrade certainly occurs. There are examples on this forum, myself included. But is that typical/common? I know I'm far from typical – I had the 70-200/2.8L IS II, the 85/1.2L II, the 100-400L, and the 24-105/4L IS with the 7D, prior to getting the 5DII.
I wonder how many Canon APS-C owners upgrade to FF, how many have EF lenses when they do, and how many of those were lower IQ lenses that were replaced L lenses after the upgrade. But...whereas I can only wonder, Canon has the data. In light of those data, there maybe no real problem from their perspective.