In the US, cash purchases became a lot less common during the pandemic for obvious reasons, and it never really bounced back. Businesses were hardly ever credit-card-only before, now it's not that uncommon, and cash-only ones are way less common than before. Maybe related, several areas raised their sales tax beyond 10%.
It is enforceable, it’s just that not all countries are there yet. It’s practically impossible to buy anything worth more than 1000 eur with cash in Scandinavia, ironically with the exception of drugs. Humanity is on a steady track to a world where authorities are controlling every penny, at which point non-sanctioned opposition is impossible.
Which is trivial to ensure when people effectively can’t deposit cash to their bank accounts and businesses don’t accept cash - nobody wants the headache. Which is exactly what happened here.
You are allowed to travel abroad with under 10 000 euros without declaring it. As far as I know we do not have the same problem with asset forfeit laws like the US. There are no laws limiting how much cash you can have at home and while travelling. Though if you hoard millions in gold that is discovered in a police raid I think your main concern will be why the raid happend.
In the EU, laws differ by country, so I wouldn't categorically declare that it's legal everywhere to have any amount of cash at home or traveling. From incidental local news reports here in the Netherlands, I suspect that if I were found during routine checks to be traveling with multiple-10k cash in my backpack or in my car, it would be seized and be treated as illicit until I prove otherwise.
