I'm not going to discuss how long any engineer needs to work on a certain project, how many lines of code he wrote or whether he's creating more or fewer bugs. I want to focus on the soft skills differences between a junior and senior.
I have an extra requirement for senior roles - it's the ability to change and improve individuals, the team, focus groups or the whole company.
If you have a true expert who is unfortunately not able to share his bright ideas with others or not able to convince others that these ideas are really bright, where is the extra mile that makes him a senior?
Besides the technical excellence, a senior may find his sweet spot in people. The great first opportunity is to let such an engineer be a buddy for a newcomer in the team or guide for a junior colleague. Btw. if these people are willing to do the career switch, they are great candidates for future managers.
In addition: in case you'll ever hear that "promoting is an art", get rid of anyone who says this. Career path must be always clear, well defined and it's simply not fair not to promote anyone who has achieved all the prerequisites.