Bitcoin has had several big moons/busts now, but the software and network has persisted and grown dramatically in size. The long term trend is up. I think it is really hard at this point to make the case that this is a bubble.
Bitcoin is an “asset”, not an intermediary. Saudi doesn’t just sit on their dollars, they buy stuff with it, either actual goods and services or fixed incomes. Since nobody wants bitcoin for their labor and it’s a highly speculative asset that isn’t a particularly good store of value, it doesn’t accomplish either of the key things that are needed.
Bitcoin is both an asset and an intermediary. I don't think it is a good intermediary due to its volatility and tax treatment, but it certainly has and can be used as such.
If you had bought a hundred dollars worth of Bitcoin back in April of 2011 and sold it at it's most recent all time low on December 16th 2018 you would have $320,000. If you sold it today you would have $681,800. Volatile? yes. Good long term store of value? All available data indicates yes.
It's speculative in so far as the software and the network are speculative. The speculative risk of both of these attributes has reduced over time. (i.e. The more time that goes by the more likely that bugs in the protocol will be discovered and the network size and volume has dramatically increased)
The real danger to Bitcoin is probably another cryptocurrency that comes along and does it's job better. This will be very hard though as Bitcoin is essentially the Facebook of the cryptocurrency world. I think that it is possible that this will happen, but it will likely be a process more than an event. Offering holders of Bitcoin to diversify overtime should other cryptocurrencies come into being.
Whether Saudi Arabia should buy it or not is an entirely different matter. IMHO if they have some reserves that they don't think they will likely need to touch for a 5 to 10 year period then I would invest some in it. It would be akin to trading in some of their dollars for Gold. (https://tradingeconomics.com/saudi-arabia/gold-reserves)
