Mercs 3D sold well considering thats all it pretty much is (mercs mode)
Also, try playing Mercenaries on RE 4 or 5 after you have played mercs on 3DS...... RE 4 & 5 mercs are terrible in comparison (bending down to pick stuff up, faffing about with the menus ect)
REveletions is only 4 months old too..... so with more people taking the plunge and buying 3DS's, i still think it will have legs.
And i know i say it a lot...... but REvelations is STILL my fave game this year (worth buying a 3DS for by itself)