Ex: The only things you can do with already mined blockchain blocks are selling them, or hoarding them.
You don't actually do anything with blocks. Blocks contain transaction data for the network, confirmed by miners who play hash games for block rewards. I think you mean the actual coins that come as block rewards, aka Satoshis / Bitcoin.
Are you familiar with decentralization and developing apps on things like Ethereum? Or IPFS?
Cryptocurrencies aside, currencies are hard to understand. It's not "you trade money for stuff, duh!". It's how USA can print dollars to import stuff from China year by year without giving anything back. It's who prints money. It's how come printing dollars makes existing ones less valuable while also creating debt. How does independent currency affect debt-based currencies?
It's easy to be elitist. We know about computers more than average Joe, so we are experts on computer's money, but IMO cryptocurrencies are more currencies than crypto. And currency part is harder to understand than crypto part.
I also doubt that blockchains contribute anything of value for most applications that are currently being tried out, but I also felt that "selling blockchain blocks" was an odd way to phrase it.
It's a bit like someone claiming that a perpetuum mobile is impossible because "movement consumes energy and eventually it will run out". You can't tell whether they have the correct idea about conservation of energy and momentum or whether they actually believe that moving at a constant speed would constantly require energy even without friction.