MKXL is basically the Arcade Edition of MKX.
On June 7, 2011, Capcom released the Arcade Edition update to Super Street Fighter 4 on consoles. It was an update that included balance changes across the cast and four new characters. It could be bought either as an entire new retail package or as an update to the previous installment.
On March 1, 2016, NetherRealm Studios released the MKXL update to MKX on consoles. It was an update that included balance changes across the cast and four new characters. It could be bought either as an entire new retail package or as an update to the previous installment.
Arcade Edition's first appearance in a major tournament was CEO, which was June 10, an entire 3 days after AE came out. Meanwhile the new characters in MKXL are, as of now, banned from use at Final Round. It was debated at the time whether going AE was the right choice with so little prep time (especially with internationals coming who had been playing it in the arcades for 7 months already), but it worked out fine. MKXL players would have six times as much time to learn the new game, in comparison.
To me, 2.5 weeks is a decent amount of time to get used to it. It's a week more than Street Fighter V players had between release and Winter Brawl, for example, and that's a brand new game. But if you want to exclude the characters, you might as well not run MKXL at all, including the balance update. It's probably more work to learn all the new stuff than it is to learn the new characters, so why go half way?
