- real estate - to live in or to do business in them
- stocks - as they're shares of a company they could be seen as the equivalent of a share of the "infrastructure needed to produce goods and services" (which, again, satisfy needs of people),
- bonds - very much the same than stocks, only that also states can use them, in this case they're are shares of other kinds of infrastructure (justice system, social security, defense etc.) which also satisfies needs.
Bitcoin can absorb a part of the "market caps" of these assets, but only a relatively small part - the part which corresponds to the overvaluation of the assets due to speculation. I expect this not be more than 30% in extreme cases, more close to 5% in normal conditions.
Take stocks for example: let's consider Bitcoin has absorbed a part of their market cap, leading to a smaller stock market. But likely, value investors would see an opportunity to re-invest in stocks because they are shares of "real things" with "use value" and also produce real cash flows like @Kyraishi wrote. Even derivatives have connections to that value, while the Bitcoin value proposition is much more abstract.
I very much agree with this quote from .@stompix:
People dump 50% of the shares in companies, land, houses, everything that makes wealth, and buy bitcoin with that money. What is the result? You will have half of the value in wealth and twice the value in currency. Guess where that leads to! Not a pretty place!
However, I disagree a bit with respect of gold, and here I'm more optimistic. Gold (ald other precious metals) is a hybrid case. It has some use value (industry, jewellry), but most of its value is speculative. Even if we concentrate on jewellery - if gold wasn't as valuable as it is, then people wouldn't like jewellery as an investment or as a marriage/engagement gift. There is also an important psychologic factor.
Imagine Gold crashes 70% because banks/central banks divest it and instead prefer Bitcoin. Some (mostly older, traditional people) perhaps will take the opportunity and buy some gold jewellery, but it would be seen generally as something "cheap" and out-of-fashion. Like Bitcoin when it's "dead" again. Young people from the FFF generation will probably prefer jewellery made of 100% scrap metal recycled 100% with renewable energies.
So yes I think Bitcoin can "absorb" a large part of the gold "market cap". Maybe not 95% of it like the OP wrote but perhaps 50-80%. But with respect of the other asset classes it's imo only a tiny fraction.