Not to get too far afield, but if you get a chance to see a really high end 4K projector you should. I have seen several of the Sony units now. They have come down in price but are still a large investment.

That said, the thing that people kind of forget when they complain about the lack of 4K native content (and I have seen some of that too, via Sony's weird round server thingy...) is that up-scaling has MASSIVELY improved. To the point where I honestly did some back-to-back viewing of plain old DVDs and then BluRay and even broadcast / cable / satellite and was BLOWN AWAY with how well suited upscaling is to a 4K projector. Today's built-in upscaling devices have a whole lot more CPU power behind them and much smarter algorithms so there are NO nasty artifacts / color smearing. It is doing much more than old fashioned "line doublers" ; the units are extracting / interpolating extra data to drive the additional pixel elements seamlessly. If you watch something like live sports (which tends to not be compressed for obvious reasons) you can do frame comparisons and you'll see that while it cannot create detail that is not there (say like making fans in the 50th row completely detailed) what is does do quite well is enhance ALL the picture elements to such a degree that you are so impressed you don't really care about insane levels of detail -- you get much enhanced foreground elements / edge control, better color fidelity across the whole image, impossibly fine grain edge to edge and very very pleasing / natural motion.

And there really are no downsides -- it is not all like the bad processing that some cheap sets still use to get higher artificial frame rates. It does not alter the response time or unnaturally reshape quick moving objects. Obviously this takes LOTS of CPU power, but if you see the smarts that are packed into game consoles / high end video cards this sort of processor is obviously this kind of thing is no longer as costly as it once was.

Do I want native 4K content? Of course! It will give the kind of reproduction on large home projectors (and the biggest flat displays...) that previously ONLY happened in the nicest 70mm type theaters. Highest level of true detail, finest control over color space, best native contrast ratio all will be as intended by the content creator, BUT as the 'engines' that drive the displays are VASTLY improved the lack of native content is not as big a deal...