I would call it more of a plateau than stagnation. The senior title is a pretty beautiful spot, well paid and always more to learn but the value to the company is limited to the work you get done. You may be able to get it done faster and better than anyone else, but there are diminishing returns on speed and quality of an individual. I miss being a senior sometimes. Being able to go heads down and pair with someone all day on a problem was a great feeling, but I enjoy how much I help my coworkers/friends.