Cardano will never have the full amount of the circulating currency delegated in pools.
This sentence, which seems negative and capricious, is actually a product of Cardano’s monetary policy design.
How much can be delegated?
Only the circulating currency, i.e. the amount of ADA issued by the Ouroboros protocol and held by the holders, can be delegated.
Each epoch the protocol expands 0.22% of what it has in reserves, and note that each following epoch, the balance is of course smaller, and therefore the new issue implies a smaller amount each time.
As of today, epoch 254, in March 2021, there are 31,959,593,112.86 ADAs in circulation (1), i.e. 71% of the maximum total of 45,000,000,000,000 ADAs that will be expanded over the years.
But not all of these ADAs in circulation today are in the hands of the community, so delegation is not possible.
Part of the ADA issuance and part of the proceeds from rewards go to the Treasury.
Thus, the Treasury is funded with 20% of the issuance and the collection of rewards from each epoch.
This means that not only 20% of what is issued each epoch cannot be delegated, because it is not in the hands of the community, but also 20% of what is collected from transaction fees (which are part of the rewards).
The treasure makes sense for Catalyst to fund winning community proposals that are voted by the community to be applied to Cardano’s utility.
The treasure is the monetary expression of Voltaire, the governance of Cardano.
Now, whenever funds are released from the treasury to give resources to the winning proposals in the various FUNDs, those ADAs can be delegated, because whoever receives those funds and spends them to implement the proposals, someone else receives them, and can thus be delegated.
Although the calculation is difficult, I can make an approximation that illustrates the idea, and it is possible from the mathematical approach to calculate a maximum delegation of 80 to 82% (depending on the amount that is decided for the FUNDs) and that arises from:
monetary expansion + rewards + funds distributed by Catalyst - Treasury funding.
If you want to understand more about monetary policy: Cardano’s monetary policy