Projectors with proper HDR will be difficult to come by because of the brightness requirements, but the advantage of 4K with front projection, as it relates to screen size is not at all in question.
At about a 1.5x screen width viewing distance (center of theater), the advantages of resolutions beyond 1080p are already visible. But, many people sit closer than that. A typically diagonal size of 10" to 12" for each foot of viewing distance is common. So, at 10' viewing, a 100" to 120" diagonal is very normal these days.
On the low end, 100" diagonal at 10' viewing, that puts the eyes to screen distance slightly closer than 1.5x screen width, and in the range where the benefits of 4K become apparent.
Of course, rec2020 color support should be part of that change, as should any capacity towards true HDR support, but that will all come in time.
As for lensing, that's a great point, but also an industry lie.
The exact same words were said when 1080p came to market. But, the reality is that they are making lenses for cameras which shoot at 10+ megapixel. UHD sits at about 8 megapixel. The Sony projector, which is native 4K, and is about $5,000 or so, is not priced out of this world, and is a very solid projector. When Panasonic added a 4K projector, they actually didn't upgrade their lenses from the WUXGA models they had before it.
So, it will be about proper lens design and quality manufacturing above all else. A well engineered lens design. I mean, even on cheap projectors the interpixel gap of DLP is clearly visible. I can't even tell you how fine that sharpness is. Perhaps 1/32 of a pixel in width, and we can get clear focus of that. So, 4k is about 1/4 of the pixel size currently, and is far less than the interpixel gap we current can sharply focus on. No, I don't think lenses will actually be the issue some make them out to be.