As a manager I would be fine with this explanation. If you are valuable in the area you are and you don't want to move on that's fine with me. We need someone in that role and you fill it nicely so by all means stay in that role. There should still be plenty of growth opportunity for you if you want it... it doesn't have to come with a title change. Having 100 people that all want promotions that cycle and only 20 slots to promote into is stressful to handle so you not being part of that but still contributing is kind of great.
