Sheldon (Jim Parsons) and Leonard (Johnny Galecki) from the original pitch, the revamped version added three more main players in Penny (Kaley Cuoco), Howard (Simon Helberg), and Raj (Kunal Nayyar).
From there, The Big Bang Theory gradually grew its base. By season 4, it added two more series regulars in Amy (Mayim Bialik) and Bernie (Melissa Rauch) in an effort to provide love interests for Sheldon and Howard. The show ran for an additional eight years until it wrapped up in 2019 with 12 seasons. Its cancelation, however, had nothing to do with its ratings or its popularity; The Big Bang Theory remained to be CBS' premiere sitcom when it wrapped up. It became so big that the studio was willing to pay the cast $1 million each per episode just so it would continue. So, it was surprising even for some of the stars of the show when Parsons revealed that he was leaving which ultimately led to the decision to cap it off altogether.
For context, The Big Bang Theory ranked top two in of ratings since 2012; it ultimately reached number 1 in its eleventh season. But unlike other cult-favorite sitcoms like Star Wars: The Force Awakens in 2015.
Aside from that, The Big Bang Theory has also started running in syndication in 2010. Through constant reruns, those who weren't initially interested in the show were able to check it out. With easily digestible narratives coupled with its accessible brand of humor, it was easy to watch. Viewers didn't need to commit to watching a full season to be entertained as each episode was fun and humorous. It's also worth noting that The Big Bang Theory’s scientific element piqued the interest of those who were already intrigued by the characters' respective fields of study. At least in its earlier seasons, the boys would either do fun experiments or indulge in their latest nerdy interests, regardless, they seem to always have a blast. Meanwhile, its eventual availability in streaming sites such as Netflix and HBO Max helped it further broaden its reach, with more people checking it out on their own time.
Given its enduring popularity, it's safe to say that The Big Bang Theory would've continued to run for a couple more seasons had Parsons decided to stay. Outside of its issues, the show still has remaining stories to tell, especially with regard to other characters aside from Sheldon and Amy. In any case, the universe it exists is still alive thanks to the prequel spin-off, Young Sheldon.