I am willing to focus on this particular point for the time being and I will attempt to be less wordy where possible, though I am hesitant to leave points out.
There is currently a BitCoin market. It is small but it exists. There is almost no barrier to entry into this market (overhead costs are minimal). Because there are goods/services that can be purchased with BitCoins, any person who sees a large enough collection of goods/services available in BitCoins can easily start accepting BitCoins as payment (along side another currency). As more people accept BitCoins, the collection of goods/services available in BitCoins increases. This increase in available goods/services results in more people willing to enter into the BitCoin market.
If we assume that every person on the planet has a certain “collection size” (number of goods/services available in a given currency) required for them to be willing to participate in that currency and we also assume that every person participating in that currency increases the collection size of that currency then we can conclude that the only barrier to mass participation is if there is a significant gap in collection sizes somewhere in the population spectrum.
If there are 5 people on an island:
| Person | Required Collection Size |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 1 |
| 4 | 2 |
| 5 | 4 |
In this example there is no gap in the required collection size that would result in a maximum collection size short of the entire population. The first person requires no one else, the second and third person require only one person thereby increasing the collection to 3, the fourth person joined the bandwagon at 2 bringing the collection to 4 and the 5th person joins once all the other 4 are in.
This is an example with a gap resulting in a permanently limited collection:
| Person | Required Collection Size |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 2 |
| 4 | 4 |
| 5 | 4 |
In this example the collection will get up to 3 people but the 4th and 5th people will never join in because they are both waiting for the collection size to grow by one more.
I believe the first example is the real world case, but this is unprovable. It sounds like you believe the second example is the real world case, which is also unprovable. This is why I think that BitCoins are possible, but not guaranteed.