Why something that has a finite supply would be viable as a currency.

Since it's deflative all the incentive is holding, not spending. Why buy a house for 5 bitcoins now, if I can get it at 4 bitcoins 3 months from now..What if I could get it at 1 bitcoin in 3 years...crazy right..?

And no one spends, because the incentive is not here.

Our economies are all based on people wanting to spend or invest rather than stock money.

Finite supply doesn't even matter anymore since exchanges are now operating like banks with fractional reserves, loans and everything. Through credit they can inflate away the value of cryptocurrencies as much as they want. The entire financial system was reinvented on top of bitcoin, it's the worst possible outcome.

Elephant shit also has a finite supply, and is actually more useful than Bitcoin.