I have recently worked with several strong full stack developers and helped them transition to new companies. There were two common themes for why they decided to make the transition. The first was challenge and the second was their intense desire to stay current.
Each of the developers told me that they had been with their current companies for over 2 years and they felt that the excitement/ challenge of being a change agent within the company had worn off. It can be difficult for a company to identify ways to challenge their people. At the same time, the cost of not identifying ways to keep current employees engaged and challenged can be high, in terms of the cost of hiring and training new personnel as well as the productivity lost with the transition.
Many strong technical people have an intense desire to make sure they stay current. As the use of XaaS technologies proliferates, it is becoming increasingly important that your key technical talent be given opportunities to expand their tool kit by either taking training or being given side projects to provide them with an opportunity to learn and share.
Keep looking for ways to challenge your people; it will really pay off.