Billboard will honor someone as the "Artist Of The Decade" at the end of 2019.
Let's have a look at the year-end charts since 2010. All can be found here:
http://www.billboard.com/charts/year-end.
2010 Top Artist
#1 Lady Gaga
#2 Taylor Swift
#3 Eminem
#4 Lady Antebellum
#5 Kesha
2011 Top Artist
#1 Adele
#2 Rihanna
#3 Katy Perry
#4 Lady Gaga
#5 Lil Wayne
2012 Top Artist
#1 Adele
#2 Rihanna
#3 Taylor Swift
#4 Drake
#5 Justin Bieber
2013 Top Artist
#1 Bruno Mars
#2 Taylor Swift
#3 Justin Timberlake
#4 Rihanna
#5 Macklemore & Ryan Lewis
2014 Top Artist
#1 One Direction
#2 Katy Perry
#3 Beyoncé
#4 Taylor Swift
#5 Justin Timberlake
2015 Top Artist
#1 Taylor Swift
#2 Ed Sheeran
#3 The Weeknd
#4 Drake
#5 One Direction
2016 Top Artist
#1 Adele
#2 Justin Bieber
#3 Drake
#4 Beyoncé
#5 Twenty One Pilots
As you can see, Adele is the only act with more then one year-end #1 positions, three in total.
Taylor Swift has been incredibly consistent, appearing in five of these Top 5 lists.
If Adele releases one more album before the end of 2019, she will very likely take home the Artist of the Decade award. Otherwise Taylor will take it, especially if her next album smashes like "1989".