So what your telling me is there is no reason why we actually need different sensors other than marketing purposes. Cause like I said before with old camera's the features not sensors is what made them unique.
I would argue that cost is a huge part of marketing, so, technically yes marketing is why, but, only in a roundabout way.
So even though FF sensors cost more, if they sold 100K rebel FF's a year I think you could spread R&D costs around and prob make a profit.
Except if they can drive 50% of the profit margin of a 5D with the Ti series, and yet sell 50x as many, they'll make a hell of a lot more money. The biggest thing APS-C (and now mirrorless) is lower the entry barrier to better photography
Honestly evaluate the average DSLR buyer. They have no idea the difference between APS-C and Full-Frame in terms of how it impacts pictures, and so they probably aren't going to see the benefit of paying 5x as much for a 5dIII over a T3i. So Canon would be losing hundreds of thousands of sales if they only offered a full-frame entry camera at say, $1500.
Sensor size can be marketing, but, it also has many, many practical applications for why its bigger on some cameras and smaller on others. Most entry DSLR users would be frustrated by missing focus with the shallow DOF of full-frame, or by their pictures looking softer because their cheap lenses don't match the image circle. Not to mention, good full-frame lenses are heavy and expensive to make. APS-C is a way to cover those flaws yet still provide much better IQ than a P+S at a reasonable price.