Those who have bitcoin would just be getting richer and richer without doing any real work.

But you can't just get rich out of thin air? All the others would be losers.

And if a lot of people had bitcoin, they would decrease all their economic activities because the most profitable thing is simply hoarding.

And this is not good, because without activity, economy collapses. We would get Great Depression 2.0.

(or 3.0. if you count late 2000s recession as 2.0)