[Ogre]

I only skimmed it, but I didn't see anyone in there who took years. They were all on the order of months. Entirely possible I missed it.

That said, adding networked multiplayer to a game is very difficult. But designing a game around networked multiplayer is not nearly as hard. Single player or local only multiplayer games can get away with lots of assumptions that completely break multiplayer, and detangling all of those assumptions is the most time consuming part of converting a game. If you do it "right" from the start, you don't ever allow yourself to make any of those assumptions and everything just works. If you know you're going to build a multiplayer game, then write it as a client server architecture from day one even if you know you're not going to play with other people for the first several months of development. Future you will thank present you.

[archagon]

In regards to SpeedRunners:

"Implementing online multiplayer was very difficult," developer Casper van Est tells me. "People always warn you about it, and even with that taken into account, it turned out more difficult than we expected. We're a small two-person development team, so we already knew implementing it was going to take a while, but it ended up taking us several years."