Sometimes you need to let talented engineers build things from the ground up, because it's good for them, makes them happy, and stops them from going to work somewhere else. Keeping people with the skills to solve these types of problems around and happy is also great for recruiting and for helping your less capable engineers learn and grow.
Sure, but duplicating Kafka? How many man-months did they put into building this, proving it out, and dealing with fallout from any bugs or production issues?
What's the point of retaining an engineer who's doing nothing for the business but re-inventing existing successful software?
"Sometimes you need to let talented engineers build things from the ground up, because it's good for them, makes them happy, and stops them from going to work somewhere else. "
actually, trying to let people do work you don't need done, in order to keep them happy, is a pretty rookie manager mistake.
If they aren't passionate about it, and you can't persuade them to do the things you need doing, they aren't the right person for the job.
That is always true, even if they were the right person in the past.
Your goal in that case should be to try find stuff the company needs done that they want to do, and push them to work on that. But if you find nothing, ...