Well, people can hit plateaus for a while. I hit a plateau of between 70 & 80wpm for a good 1000 games or so, but over time it was overcome and exceeded.
Some people may hit longer plateaus than others, I'd say, and not everyone would be continually persistent.

Why is everyone on this site not trying to improve at all? Ok not _everyone_ is like this, but so many people's WPM score is completely stagnant. What's the point of coming here if you're not trying to get better?