"but its also true that the reason of fees to be low is low usage"

Do you mean on Monero? This is not true, that's the whole reason for dynamic blocks in the first place. We even have real world examples of it in action very recently on Monero. But continue believing this if you like 🤷‍♂️ 

 Its true unless someone has succesfully conducted spam attacks and bloated the utxo set instead of having fees spike. Its true what you say, but still after the attacker has been bored the usage has drastically lowered to a real-usage level. 

 If what you're saying is that a similar spam attack would bloat Monero faster than Bitcoin, then okay, that's true, but nothing to do with fees again. Different topic.