The Big Bang Theory co-creator Bill Prady admits that Penny and Leonard almost didn't end up with each other. 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.
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. We said they legitimately could be the person for each other and we were rooting for it, but they had a lot of obstacles.
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.