Additionally, the software could be forked to allow division of BTC to a more granular amount than 0.00000001, meaning if all but 1 BTC were lost the entire world could still transact with that last singular BTC by dividing it up into smaller units.
A little bit like how a 'Calorie' is actually a kilo-calorie.
It could be that all those lost bitcoins are just sitting in someone's harddrive, waiting for moment to crash the market
The system for trading ownership of bitcoins does not care for the value of that ownership.
The value we, outside of the system used for logistics of trade, place on the bitcoins is then affected by their scarcity.
To the system, it doesn't matter if the scarcity is caused by loss or by hoarding. But this can affect our valuations.