The Big Bang Theory co-creator Bill Prady admits that Penny and Leonard almost didn't end up with each other. Although Penny and Leonard's relationship took a lot of unexpected turns, it was clear that they were The Big Bang Theory's primary romantic pairing, hence the expectation that they would ultimately end up with each other. The hit CBS series ended with them happily married and expecting their first child. However, The Big Bang Theory wasn't always set on these two opposite personalities sticking together.
Prady opens up about this matter in Jessica Radloff's book, The Big Bang Theory: The Definitive Inside Story Of The Epic Hit Series. Comparing Penny and Leonard's story to Friends' Ross and Rachel, the producer reveals that the Hoftstadters weren't exactly planned to be endgame on the sitcom. In fact, the creatives were open to the possibility of moving their narrative forward as a one-time couple who eventually became friends, rather like how Johnny Galecki and Kaley Cuoco's real-life relationship panned out.
After an exhausting first season, we came back in June and started up the writers room; just having conversations about where the season could go. I think one of the things that I ultimately came to embrace about Leonard and Penny as a couple is that they had a rough go of it, they broke up and got back together, and there were periods where they saw other people. It didn’t feel the same as Ross and Rachel’s relationship on Friends; I always believed their relationship was written as, “These people are inevitable partners.” And I don’t think we ever wrote Penny and Leonard that way. We said they legitimately could be the person for each other and we were rooting for it, but they had a lot of obstacles. So when they’d break up, it wasn’t false jeopardy for the audience. Them not winding up together was an absolute legitimate path forward for the series, and it could’ve been. They could have easily been that couple who once went out and now are friends.”
What The Big Bang Theory Could’ve Looked Like Without Penny & Leonard’s Romance
In the same novel, it was revealed that Penny was the reason why Leonard's other love interest, Leslie, was written off The Big Bang Theory. As it turns out, viewers immediately became very invested in Penny and Leonard's relationship, so the writers decided not to continue using Leslie as a third factor. Given this, it's highly likely that the public's reception to the storyline significantly impacted the writers' decision to move forward with their romance. Granted, they dealt with some rough patches throughout The Big Bang Theory's 12-year run, but there was always an assumption that they would find their way back to each other.
As one of the most popular aspects of The Big Bang Theory, it's difficult to imagine what the show would have looked like if CBS pulled the plug on Penny and Leonard's affair. One thing that is for sure, however, is that it would significantly change the dynamic of the Pasadena gang. Although Penny and Leonard learned how to become civil with each other when they were broken up, there was always a romantic aspect to their relationship. Other than that, Penny is the only person who could make Leonard truly happy because she, too, genuinely cares about Sheldon despite his annoying traits, just as he does.
Sadly, The Big Bang Theory ended up neglecting Penny and Leonard in the end. After their wedding in season 8, the pair's collective arc was stagnated as the sitcom shifted to focus more on Sheldon and Amy's flourishing relationship. The Big Bang Theory's final season attempted to rectify this by introducing a new conflict about Penny and Leonard's opposite stance on having kids, but it was controversial. Many felt it wasn't handled well and became a disappointment. In fact, even Cuoco admits she doesn't like Penny's pregnancy reveal.
Source: The Big Bang Theory: The Definitive Inside Story Of The Epic Hit Series