Sheldon Cooper problem. In 2018, shortly before season 12 debuted, it was announced by CBS that the long-running show's year would be its last. It was a massive shock since there were rumblings about greenlighting season 13, not to mention The Big Bang Theory's enduring popularity.
Eventually, it was reported that the series' cancelation was due to Parsons wanting to leave the project. He was part of the show's original cast, playing the socially inept genius Sheldon for more than a decade and he was ready to move on. People involved with The Big Bang Theory at that time were quiet about the matter; they were mostly focused on delivering a satisfying last hurrah to the Pasadena gang. While The Big Bang Theory wrapped up in May 2019, some of the cast came out explaining the sitcom's seemingly abrupt end.
Parsons ultimately itted that his choice to leave winning the Nobel Prize in Physics. This issue was the most explicit in The Big Bang Theory's final season as it was primarily devoted to Sheldon and Amy's work on Super Asymmetry.
Granted that the remainder of the characters were given something to do, their respective arcs were mostly B plots. For example, at the beginning of season 12, Raj's love life restarted with the introduction of Anu; for a while, it seemed as if everything was well for the couple. But in the middle of the year, this narrative was barely focused on, until it got totally lost in the mix. In the end, The Big Bang Theory decided to simply break them up since there were no significant developments to their relationship. Meanwhile, Penny and Leonard's pregnancy was set up to be a major storyline, and for the first time in a while, it gave the characters a significant arc. But, the series didn't have enough time to properly tackle a very serious conflict, that in the end, its resolution was rushed and even off-putting. The Wolowitzes' life was stable, but the show left one major mystery regarding them: What really happened to Howard's dad? Given all these narrative opportunities, the show could still use at least one more year to address them.
Sheldon experienced tremendous personal growth over the years. He learned how to have a sustainable relationship and he had mellowed down some of his worst quirks. At the end of The Big Bang Theory, he was happy personally and professionally; no wonder Parsons felt the character's arc was complete. But it was an ensemble sitcom, and it would've been great if every character in the show was given the same satisfying ending.