If they didn't care they would have left Dota 2 running on Source 1.
do you know how long the targeting bug and the hitbox stuff is in there? not to mention the fps drops. they care more about the next compendium than the game being in a playable state, so unless multiple pros come out saying something that wont change anytime soon.
and at least source 1 was playable, so i'd vastly prefer if it was that engine still.