What prevents the organization of X number of rich donors, who spend some of their time and money soliciting pledges from other donors for an Alcor endowment, that Alcor only ever sees if it signs a contract to spend the donation on a endowment and nothing else?
Money is fungible. If you distrust Alcor that much, why do you expect Alcor to not, say, immediately cut additional funding to the endowment to a minimum or 0; or run up debts expecting to repay them from the endowment once the period expires - etc. etc. etc. This is a classic principal-agent issue and why people want to signal honesty rather than just signing detailed contracts: there are many ways to cheat or violate the spirit of an agreement.
One of the sticking points for cryonics is how expensive it is. Unfortunately, the estimates on LW (eg. in Normal Cryonics) are likely to be low as they are current costs. This is starting to come to a head for Alcor, with Alcor's low growth rate meaning it faces a rising tide of aging members (hence that emphasis on young cryonicists) and fundamental flaws in its prices; the official word has come down in the latest issue of Cryonics, issue 2011 q4:
Cryopreservation Funding and Inflation: The need for Action; A Discussion Article by the Management and Board of Directors of Alcor
What to do?
Hope the old grandfathered members like Mike Darwin (who predicted this, in the February and March 1988 issues of Cryonics) can afford that.
On a parting note, I read somewhere that CI's low prices have rarely risen. I wonder what their projections look like...