Eric has published several other significant works as well. And the museum is to a large extent his collection and library plus other exhibits. The 23 million his collection has brought so far is just from the sale of I believe his duplicates. (although they may be the better duplicate example)
The real question is what will happen after Eric dies. I'm reminded of two other collectors who left their collections as museums/research facilities, Byron Reed and Harry Bass. Reed left his collection to the city of Omaha on the prevision that it be on permanent display in the museum. After a break in attempt the city sold off a lot of the better stuff (with the promise that they would be replaced with lower grade specimens.). The money was used for other purposes in the museum and city coffers, the coins were never replaced and the collection is now locked away out of sight in the vaults.
In the case of Harry Bass, he created and endowed a foundation to create a museum and facilities where researchers would have access to the coins for research. After he died the foundation trustees sold off everything except the core gold coin collection (And I think part of that was sold as well) including the extensive research library. Then they gave the gold coins to the
ANA Museum on indefinite long term loan on the provision that the
ANA create a gallery to display them. This dumped all the expenses for storage, display and insurance onto the
ANA and off the Foundations hands. The Foundation still works with the ANS some but the major expense for the foundation and the endowment now is paying the trustees salaries.