I'm just pointing out that even with not so many transactions, you don't simply "die" - and evidence of that is the marketcap and mobility of gold, because bitcoin is not simply a payment system, but also a store-of-wealth system.
Gold is a store of wealth because it's a metal with very useful properties. If it were cheap everything made out of copper would be made out of gold instead, but there isn't enough gold to do all that, so it can never be cheap.
Bitcoin is only a store of wealth as long as it's the coin everyone uses. Intrinsically it's no better than the hundreds of other coins out there. If you cap it, it will no longer be the coin everyone uses, because caps by their very nature exclude people. So its value will fall, and whatever altcoin people start using instead will become the new dominant coin.
AlexGR is correct that bitcoin cannot scale to cover the entire global population, but it can scale much bigger than it is now, and if it does not, some other coin will.