The entire Cryptocurrency market cap is relatively small when compared with the worth of certain large traditional financial institutions. In theory, a wealthy individual could purchase all Cryptocurrencies if the current market cap is the only thing taken into consideration.
The reality is that any attempt by a single entity to purchase all Cryptocurrency in existence would cause a very sharp rise in price which would incentivize more people to hold on to their crypto assets. As a result the “wealthy individual” would not be able to buy all of the Cryptocurrencies in a short period.
It also has to be said that there isn’t much to gain from owning all Cryptocurrencies in existence since the number of users of the Cryptocurrencies has an effect on the value of the crypto assets.