What happens if I find an old chest with a 100kg in gold/diamonds/unobtainium? I think this situation is quite a good analogy to your question, the consequences of which are readily known.
I think the 'point' here is that as opposed to chests with 100.000.000KG of gold there are actually 'lost' wallet files with enough BTC that it would 'rock' an economy with a currency cap in the future.
I am pretty sure if someone found a stash of gold worth $400.000.000.000 that WOULD actually effect gold prices. Also: Gold has 'real life' uses (production of components etc) and real world costs etc. attached to handling (weight, transport costs etc). None of which BTC has.
So I don't know how much gold it would take to rock the price, but I'm guessing it's enough as to be non-trivially "lost" however, any random flashdrive could contain any number of btc.
The analogy to gold isn't gold that was once possessed that has been lost, it is gold that is newly discovered. A new find of a big gold vein, gold on an asteroid, etc. would be a similar shock to the system.
I am pretty sure if someone found a stash of gold worth $400.000.000.000 that WOULD actually effect gold prices. Also: Gold has 'real life' uses (production of components etc) and real world costs etc. attached to handling (weight, transport costs etc). None of which BTC has.