It has a decent chance of being so, but we should wait and see the numbers of a surprise hit CoD or the new GTA VI. Anything can happen.
Also, food for thought, the closest case of this happening until now is the GB and the GBA. Pokemon RB was the best selling title for the GB, and their remakes were the second best selling games for the GBA.
I can't really seeing anything toppling it now. Even if there is a surprise hit COD it's only really going to have 1 year of sales before it's legs are cut off by the release of the next COD, meanwhile GTA consistently places well in charts year after year and is just going to keep selling. As for GTA 6, I kind of doubt it will even be a PS4 game. Red Dead 2 will release this year and I think they'll be at least a year, if not two, between that releasing and GTA 6 being announced. Then there's probably going to be at least another year from announcement to release (with a high chance for delays) and you're probably talking 2020 as the absolute earliest for it, but probably later. PS5 is surely going to be releasing around the same time, even if GTA 6 doesn't make the jump to PS5 it will release so late in the gen that it's sales will likely be cut short be an eventual PS5 port, especially now that people know Rockstar will release one, so it won't have anywhere near as long to sell as many copies as GTA 5 has by then.