I agree with asset != investment. BTC is interesting, because it's a virtual asset; not only does it not produce an economic output itself, it also cannot be used for anything, unlike traditional assets.
The reason you could say it's a pyramid scheme is BTC, despite its creators best intentions, doesn't work as a currency. Right now it's just an asset, and the transactions done with it are similar to bartering or swapping gold bars. At the end of the day, people transfer BTC back to $ (or any fiat currency) in order to perform their other day-to-day transactions. The thing is, to exchange BTC back into $, someone has to be willing to pay $ to get your BTC. As the price of BTC increases, a smaller and smaller # of actual BTC gets exchanged, for the same (or a higher) nominal value in $. What ends up happening is very early adopters sit on a massive mound of BTC, or at least a fair amount of BTC, that they can't cash out unless someone else is willing to pay. For BTC to have "real" value, which in our world is a fiat currency backed by the government, it requires new people (arguably new gamblers and suckers) to pay out the original owners.
That's why I also think of BTC as a pyramid scheme, it produces nothing, and for it to have value it requires a stream of new people willing to pay for it. It's not that different from gold since we have way more gold reserves than can be used industrially, but at least gold has some uses. If we stop mining gold entirely, at some point we will use all of our gold reserves (unless we recycle gold 100%), and will need to mine more. This will never be the case for BTC - BTC is a bad solution to a problem that doesn't really exist.