If it makes your design friend feel any better, I've had this same conversation with people who are moving from all kinds of individual contributor roles into leadership roles.

There's only so many hours in the day, so you have to decide how to spend your time: hands-on working on a deliverable directly, or supporting and steering a team working on that deliverable.

To be blunt: if you want to be promoted/paid more, you can't keep doing what you've been doing as an individual contributor. The impact of that doesn't scale. This is a source of frustration I've seen in high-achieving people. They want to be promoted, but they want to keep doing what they're most comfortable doing.

Another source of tension in this scenario is: The high achiever is very skilled and efficient, because they've been doing it for a long time... but the company needs other members of the team to build their skills too. That high achiever needs to get out of the way, stop hogging the most high-profile projects, for the younger team members to grow too.

If you want to continue doing hands-on work as an individual contributor, that's great. But you can't also expect to be promoted/paid significantly more past a certain point. And you can't expect to always get the plummiest projects, even if you're the most proficient. If you can be at peace with that, then you can lean into your expert, individual contributor role and really enjoy doing what you love.