i am playing HS actively, and i honestly think, HS may not have generated them as much income as d3 as of now, but the potential is way higher.
given that HS follows WoW lore, there's way too many expansions and raids that they can release. not to mention that HS is currently the first and most successful online TCG (imo). it's a breakthrough for this genre of games. old school TCG like MTG earns from sales of the booster packs and some cards are worth alot in real value (e.g. black lotus). however, since it uses real cards, it is very inconvenient to have a good match due to difference in locations. game meta also changes slowly because there's lesser amount of games played.
in addition, the computing and development behind a TCG will be way more easy than an ARPG, be it graphics, debugging, etc etc.
i believe the big bulk of income from d3 was actually the vanilla release (big big hype from the long wait since d2) coupled with the RMAH back then... ever since, they removed the RMAH (bad choice imo), they have lost the income to continually earn from the said developed game. it became a one-time ROI instead of a recurring ROI.
if we take into account the new age gaming community, players nowadays prefer to play something, for extended periods of time that requires a certain level of skill, instead of mindless walking/farming/shooting etc etc etc.
in time to come, HS will definitely catch up with blizz on income value imo...