how would you not look over UAC when it pops up every time something opens? it can't be used as an indicator to spot something malicious.
and smartscreen only works on executables downloaded straight from the internet..
Using UAC is no different then using a product that prompts the user for choices. It intercepts, and you have to determine. UAC also has different presented formats/prompts, not just would you like to allow this.
If you would like to see something you would not expect, place UAC at max, windows defender on, download a pack from the Hub, and open it, then launch what is left, and tell me if Smartscreen intercepts. If you really want to test it, go to Hybrid-Analysis or similar site and download each sample one at a time with all enabled. Use MS Edge as your main browser while doing so... Should be interesting the results you find. I have done it many times, and have seen serious marked improvement over time.
As you stated, the mark of the web, certainly helps the chances of them being intercepted, it also happens to be the most common method of downloading and executing any applications for common users.