I think the “devolving into will they / won’t they” is orthogonal you the number of seasons. Many American shows are often set up around this plot line from the beginning—it’s a reflection of the vapid quality of American television in general (I blame the LA monoculture in particular). It’s not inherently a devolution; it’s just a “safe” plot line that most shows are unwilling to stray from. I have no doubt that a talented writer (who is willing to step outside of the Hollywood cookie cutter) could make a good show that lasts many seasons with many episodes per season provided the studio affords them the freedom to stray from the mold.