Bitcoin holder can benefit from bitcoin only if someone voluntarily chooses to buy it. [...]
Bitcoin doesn't grant its holders the entitlements, which is why these holders can benefit only from the entitlements (dollars for e.g.) or need-satisfying goods that are brought by new members.
Nope! Read my post again, particularly the last part.
If you define that a "benefit" only can occur if there is a price increase, then you're right, then you need new members (or more precisely: more "value" flowing into the system, be it "new" people "buying" BTC, people that already hold BTC "buying" more BTC or people "trading" it for goods and services). But again, my point is: Bitcoin
doesn't depend on a price increase.
As I wrote, the goal of Bitcoin is to become "peer-to-peer cash". This can only happen if it enters a period of relatively stable BTC price, and that Bitcoin isn't traded primarily to fiat, but to goods and services. Then a circulation emerges where value is flowing from member to member, like it flows in the current fiat-based economy, or in barter systems, which are the best medium to compare Bitcoin to. In this phase, an
increase of the value flowing into the system wouldn't be needed anymore, nor are new members purchasing it.
Where you're right is that nobody can
guarantee that this will ever happen, while Fiat provides some guarantees like you correctly state (but look at cases like Venezuela, where fiat is regarded inferior than Bitcoin in it's worst bear markets). This is the risk if you buy Bitcoin today -
it may simply fail, because it doesn't have an easily calculable "intrinsic" value.
However, there is also a chance that the "stable period" could happen at much higher prices than today. And that's why people are buying Bitcoin now and speculating to get rich.