While I told him that Steem rewards participants, he pointed out that there are people (he believes a lot but no data) who does NOT want think about money in their leisure time.

The psychological cost of making payment decision is removed in Steem. Users just upvote whatever they like and blockchain does the allocation of rewards automatically. It's true that users are going to think about money when they use Steem (because they see the rewards), but I don't see that as a big problem because they don't need to do any decisions to spend money. They can give tips to writers if they want, but that's not required in any way.

Also there are a lot of people who like to think about money in their leisure time. Just think all gambling and games where you can earn some kind of points.

If you think Steem from a perspective of gamification, it's actually simple but interesting game where you can earn points that are valuable in the real world. It's intellectually stimulating (you can read and write about interesting topics), strategic thinking is required (if you want to optimize your voting rewards, you need to think how to vote most efficiently), you need to be fast (your reward is bigger when you vote popular post first), etc. Users don't need to think about gamification perspective, but they can if they like. It's possible to use Steem as a normal social media platform and not care about rewards.

But if there really is a lot of people who would like to use Steem but don't want to see anything related to money? Well, Steem is just a platform. It can be used from different kinds of UIs. It's easy to hide rewards from UI for people who don't want to see them. Those users would only see up and down vote buttons and nothing else.

BTW, your professor might want to read the Steem whitepaper to understand better how the system works.