Years from now, the online multiplayer wouldn't serve any purpose. When an online community dies on an arcade game, those features might as well have never existed.
However, I can still grab my 2 closest nearby friends, pull out the SNES / Saturn, and play Multitap Bomberman, years after the games have come and gone. (Our local anime con has Saturn Bomberman as the main multiplaer game every year, displayed on a huge projector...) I'm glad to see someone admit that for once... I find it pretty sad when online multiplayer is added to arcade games, with no respect for offline, when you KNOW the online lifespan is potentially very small for such niche games. Great when you have map-creation and multiplater-focused bonus content with it (like Capcom's Age of Booty), not so great when you ignore the offline side, just to cram online into the game (Outland's online exclusive Co-Op.)
It's pretty similiar to what happened with Skullgirls; online play isn't an easy slap-in to a game, and when you have limited resources, it's even harder to justify. SG's started with online being a major priority, and even they only seem to have 1 network guy. And he couldn't get in every feature they wanted (spectator lobbies!), by the time the game released. (Even now, after release, getting in network features is much harder for them than, say, fleshing out the training mode, and patching the gameplay.)
Spelunky here didn't even have a network guy, and the programming guy they have seemed like it was hard enough to code the complex interactions of objects for the game itself, into their new engine. Online would have presented a whole new set of problems. It could have very well have changed the feel of the controls offline, even, in an attempt to give moves enough buffer time for smooth online play.
So far, I think they easily made the right choice. Solid offline multiplayer is much better than half-baked online multiplayer, or none at all. It'd be nice to see it patched in (once the game has made some money, like around the time of PC release), or even added as a paid DLC mode, but that's normally unlikely.