Not only that, but the speculation that the new show will provide the opportunity. The Young Sheldon finale took place in the future, Sheldon's and Amy's kids are already participating in sports and (dance, acting?) where they didn't even exist yet at the end of Big Bang. The new show will be roughly 7 years after the end if Big Bang and X years before the Young Sheldon Finale. So, even if Leonard was dead, he probably won't yet be when the new show starts.