Is this idea feasible?
I don't know why people think that having local multiplayer means adding online is just dropping some lines of code. Yes, in theory multiplayer might be feasible to give as DLC but I really doubt they would do it. Too much effort for such small returns.
I honestly think that's the main reason, while millions selling franchises like Mario Kart or Smash do give Nintendo the monetary incentive for online, a franchise like pikmin doesn't. Sucks for the people that wanted online, but this is Pikmin, the game was really never about multiplayer anyway. I never touched the option in Pikmin 2 now that I think about it.