If the two other

where already discarded, I'll definitively discard

.
In this case, the dragon pair could not be improved and would forbids tanyao.
But if they are still in play, yakuhai is worth just as much as tanyao and is more flexible (use only one set VS the whole hand).
By discarding

instead of

you're moving away from tenpai, so it should provide some compensation.
Here, I don't see much improvement.