When a senior developer is assigned a project, they usually try to figure out how the whole application works from the front-end to the back-end and maybe some of the server stuff. A junior developer typically doesn't consider these things unless they become an issue.
So if you are not a full stack developer, there is no way you can be senior developer. I have spent my last 25 years for nothing.
I think you have real big misunderstanding of seniority in this industry, like many others. Being senior developer is not just about knowing all aspects of the application, or problem solving, or being that awesome coder can fix everything with single hand eyes closed. It is not has nothing to do with how many years you spend. There was a post shared here a while ago. Being senior is about mentoring others, helping individuals, juniors, mid levels, other senior, team, company to grow. Most senior developers doesn't even code most of their time. They are the architects, brain behind the scenes, go to people in team. They will know both engineering and business side of the project. And the list goes on.