Web development is like an entry point for programming these days, the lowest hanging fruit if you will. I would not exactly call anyone who does exclusively web development "senior". It's like being a nurse - I suppose you can become a senior nurse, but you're not going to be a doctor. There's only so much you can progress professionally in web development, and each new stack gets easier to pick up, almost no challenge, patterns repeat and boredom sets in. I started from website development, then went to enterprise web application development and now moving on to data engineering (data mining, machine learning).