After my experiences with my 600D the full HD resolution gives results which are technically far above movies made with Super 35 cameras. Viewing the smaller HD movies on an XGA beamer (1024 pixel horizontally) gives crisp quality at 2.5m projection width and 5 meters viewing distance. 4k Beamers are far away and just GOOD HD beamers are in the 5k€ region. So full HD in GOOD QUALITY will be sufficient for nearly all applications where the movie will tell us a story.
4k IMO is a new idea to make things incompatible and to sell new gear and to make things more complicated - with some exceptions.
You think 4K is just to make things incompatible and sell more gear? What about megapixels on cameras then? Is that all a big scam too? Did you think the same when it went from SD to 720p? Or 720p to 1080p? 4K has alot of advantages and is the future, it's just following a natural progression of things getting better like any other product.
And while the 600D may be fine for you, there are plenty of people that want something more. Canon DSLR's aren't even doing real 1080p, there is plenty of room for improvement.
1080p is IMO sufficient for 99% of all applications and if 1080p beamers in GOOD quality are roughly 1000 EUR/$ we will wait another 10 years.
The way from SD via 720p to 1080p made sense: Image width: 4m, viewing distance: 5m, pixel size: 2mm x 2mm - that makes sense because the eyes resolution is in that region.
You are right that 1080p misses 1080 real lines. IMO it would be a better route to get REAL 1080p before you increase the sheer number of pixels without transporting more information. Nothing against 4k but it should not be the next step before 1080p is ripe.
Resolution for photography is a different thing because a landscape photo is browsed by our eyes for a longer time, resolution is important - in movies the camera operator does that for you.
But the 18 MP of the 600D do roughly compare with the 10 MP of the 40D: Limitations of the lenses and especially sensor noise cancel the advantage of 80 % more pixels a little bit. Except in situations where you have perfect light, perfect subject and perfect aperture values.