While it may look one way to customers, the reality is customers are surprisingly ignorant. (No offense intended to any readers) Most who shop at Target and buy an R100 don't really understand the difference between APS-C and Full Frame. So, it actually ends up being a point of frustration for many when they put an APS-C lens on a Full Frame camera and whoopsie... the Full Frame camera isn't working as expected!! That's why they didn't allow that on EF mount. It's never been about manipulating the market but rather protecting people's feelings. While customers naturally want specific lenses for APS-C, the actual unavoidable truth is that those lenses kind of suck because they can never cover a Full Frame sensor! There's no upgrade path path for an APS-C only lens. On the other hand, if customers buy an APS-C camera and a full frame lens... there is a clear upgrade path! That is the kindness of Canon at work.
Theres just no business case for 1st party crop glass. We kind of saw this in the DSLR days. 3rd party brands were able to undercut 1st party brands on price by like 2/3rds. Partially prob due to accepting much lower margins, but also sheerly by economies of scale. Much easier to design and sell a low price low margin lens for 3-4 systems instead of 1. This is the path forward for crop systems.
The reason that the lens mount for EF-S (APS-C) lenses was not compatible with full frame bodies is that the EF-S lenses, especially the wider angle lenses, had a rearward lens element(s) that protruded into the mirror box. The mirror design in the crop camera bodies, starting with (I think) the 20D or maybe 30D had a design where the mirror would move rearward away from the rear of the lens as it flipped up to take the photograph. Full frame bodies had no such mechanism - a full frame mirror was larger and did not have an articulating mechanism to back the mirror out of the way of the extended protrusion of the EF-S lens.
