That pretty much makes the game entirely useless once the online community for that games dies. Sorry but, I do not think it is worth $60 and I certainly did not pay that much for UT2004 back in the day. (That was the last multiplayer game I had fun with and Wolfenstein: Enemy Territory being the other.)
LOL, so?
A multiplayer game becomes useless when no one is left playing, yeah that's kind of obvious.
Let's say they did add some hokey 6 hour long single player campaign. Is that supposed to help the game 5 years from now? How many times do you need to replay a short run of the mill singleplayer storyline with no side missions or branching story paths? It's not as if any SP campaign they might have included was going to be the equivalent of an elder scrolls game.
If you really want me to, I can point out two MP focused games that suffered for months after launch thanks to time being wasted on a useless SP campaign no one cares about. They're called Battlefield 3 and 4. BF3's campaign was a joke that no one cared about a week after launch, and the MP was a mess for months. Instead of learning their lesson, we got another hokey SP campaign that was only played to unlock a few weapons and promptly forgotten about, while the MP is still a broken mess 4 months later.
Just as I wouldn't want a pointless MP mode tacked onto a game I'm buying for the single player experience, I don't want a pointless SP mode tacked onto a game I'm buying purely for the multiplayer experience.