Magnus can afford to skip any tournament he wants, but if doesn't participate, the tournament is a much worse one. One could argue a tournament where the best in the game doesn't want to participate is a lower level one.
He is the reigning world champion and has won 5 out of the last 6 world championships in Blitz. By what criteria is he "not the best at Blitz"?
[edit] You might be referring to rapid, as you mention he was doing poorly before he dropped out. However, he was also the defending rapid world champion and won 3 out of the 4 last championships there (excluding the one that just finished that he dropped out of). He was indeed not doing great in that tournament, but he was still in the running to possibly win it.
