It depends on your hobby project. Google's free tier gives unlimited map views, commercial or not. (Edit: On native apps.)
Mapbox's free tier only gives 50k map views, and only for non-commercial apps. If you're a paid app, you also have to pay $499 per month on top of your usage.
That's probably a good business strategy for the future of Maps. The fewer clients you have, the more focused you can be in terms of product development and pricing. This is true in many businesses, actually.
Every business decision carries risk. The question is whether or not the risk is acceptable.
If you don't want to serve any customers making under $10M, then other companies serving those customers are not your competition. At least not today. Is there a risk they might take their broader customer base and use it to compete with you upmarket some day? Yes. But that's why you do things like build a brand, relationships, technologies, and other competitive advantages within your niche.
Fancy steakhouses generally don't worry about McDonald's. (Not that this is a perfect analogy to Google vs MapBox. Just speaking generally here in response to your general point.)
Sounds more like Google is turning it into an actual business. I assumed that MapBox was making some money, given that it relies on OSM and can cut corners, but it isn't.
Google maps is no longer good for hobby projects as the free tier is so low.
It's like they're trying to drive away everybody but but big enterprise shops.