No matter how good your eyes are, you can only go so small without stuff becoming blurry. Personally 15"/1080p is about as small as I can go without text becoming unreadable without HiDPI scaling, and some of my friends complain about reading text on my laptop as it is.
So, for me, switching to a higher-DPI screen is not an option since it just means a higher cost, awful blur for the many applications that don't support DPI scaling, and somewhere between no and very diminished benefits. At the same time I'd never want to give up on all the extra screen real estate so 17" (or bigger!) would inevitably be the only option for me.
Also, preference and I'm not a huge fan of being so loyal to one company that they dictate my preferences. Been s pretty loyal Apple customer for laptop, phone, wife's computer, parents and iMac for since about 2005-2006. Between things like reducing their product line, not updating machines, not letting me set my own DNS on my phone and making political statements via their software the luster has worn off quite a bit.
For example, my tolerance for small text at 1:1 is about 160 dpi (4K on a 27" monitor), which is much lower than the 220 dpi of your Retina 15. Having said that, my eyes would have a better chance of tolerating the same 2880x1800 resolution on a 17" monitor at 200dpi.
At some point, that extra resolution is going to get wasted on a small screen if you're more interested in real estate over crispy retina text. I have a 4K 15.6" laptop, and there's no way in hell I could tolerate it at 1:1 with my aging eyes.
If you want the absolute screen size: indeed, I doubt there will be another. But if it's all about the pixels, you can already do better than the 17.