Here is a comprehensive list of the Billboard Hot 100 songs from the Digital Downloads era, 2005-2015 (NOT the top 100 downloads though) This cuts off about midway through 2015, so "See You Again" isn't on here. Hopefully this will give us an idea as to how Billboard plans to rank the All-Time hits next time. Note: The other Hot 100s that claim to go through 2015 don't, as UF simply says 40+ weeks on the Hot 100. So at least by this methodology, UF will pass Smooth for the next all-time Hot 100 list.
Uptown Funk is already #2 right behind Smooth on 1995-2015, and that not through the end of 2015. So especially now with UF bouncing back into the Hot 100 (at least for one week), UF will pass Smooth as the biggest hit of the Soundscan Era (November 1991-Present). And yes, it's a 15 minute video, so it takes a little time to watch it. If anyone has time to type this out, that would be even better. I think when 2015 finishes out, See You Again, Blank Space, Sugar, and Trap Queen will all make the list. All of these songs hung around long enough into 2015 to rank within the Hot 100 when it's finished.
What was the methodology? The order is completely weird
It's based on chart performance. The survey only goes through around 20 May 2015, so there are a few changes in the decade chart published two weeks later. Hopefully they will put together an updated chart soon.