"You might play something else on a phone or a tablet, for a shorter time, which could affect MMO player numbers, but it's impractical to make MMOs that are playable for any length of time on a hand-held device."
He adds that MMOs "could do with some disruptiveness."
"They're far too similar these days, and would benefit greatly from a shake-up," he says.
What really interests him at the moment is that the approach of the point where it may be more feasible for individuals or smaller groups to build MMOs, or "sub-MMOs", without the massive investment currently associated with the genre. "If we reach [that point], then we might see something special happening," he says.
"With textual worlds, people used to make bespoke games from scratch every time, until the idea of 'codebases' caught on. In the 1990s, anyone who wanted to make a MUD would take a codebase and adapt it to their own tastes. Okay, most of the results were rubbish, but some weren't - and are still going strong today.
"If people can adapt an MMO to create their own, pocket universe, we might see some real creativity going on - not least by professional MMO designers, who to some extent are constrained at the moment by the fact that MMOs cost so much to make that they can't do anything remotely risky with them."
Lowering the cost of entry for publishing multiplayer online games could help realise some of the untapped potential of the genre, he adds.
"It's just so expensive to make MMOs that people can't take risks," Bartle says.
And in the meantime MUDs, including MUD2, the successor to MUD1, keep ticking on - with user numbers that fall far short of WoW or Eve Online but a base of players that remain loyal to their virtual worlds.
"MUD2 is still alive because people still play it," Bartle says. "It does get new players, but not very often: none of the people reading this are even going to try it, because 'it's text' and they don't do text. The reason it survives is because it's free, it doesn't need many players, it has very high retention and (I like to think) it's actually pretty good.
"Last year, I had 30 of my students play MUD2 as part of a laboratory session. They had a blast, but only one of them actually continued playing afterwards and is still playing today, six months later.
"People who break through the interface and see the world behind love it; as with all textual worlds, though, most people don't break through the interface."
"Modern MMOs still have a long way to go to match [text-based MUDs'] depth and inventiveness, but text MUDs themselves are still advancing," Bartle says.
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