Text and the OS will obviously be optimized. Look how sharp Safari's nav bar is! While most assets on screen look like crap if you fill more than half the screen with the browser window. And third party apps will take forever to update as usual. Have you forgotten that there's hardly any 4K, viewable media, period? Seems silly to want UI chrome look pin sharp, but have no concern for the content that UI is there to present.
Chicken and egg problem. Would you rather NOTHING be sharp, than there being technology to both display MOST thing sharp and ENCOURAGING content to be produced for higher resolutions?
This is how we progress, not in complete jumps, but in fits and starts. Technology driving content and so on and so forth.
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skip upgrading RAM through apple, buy cheap RAM, save up to $600. good deal
If you don't upgrade to 16GB, you'll be stuck with useless 4GB chips you need to sell or get rid of, instead of 8GB chips you can augment with more 8GB chips.