I have a slightly different take on the current purposes of the endowment.
When the community was discussing the setting up of an endowment several years ago, I was one of those involved in our GLAM outreach who saw a big opportunity. At some point the endowment would be big enough that the WMF would be able to promise the cultural sector that Wikimedia Commons and or WikiSource would be around for the foreseeable future.
For those of us who talk to museums, archives, libraries and anyone else in the cultural sector who has invested in digitising content, one of the big issues is future proofing. How can I deposit a digital copy of this material in such a way that it is likely to survive for the use of future generations. Whether or not an individual cultural organisation survives in the longterm, the ability to upload a copy of their digital collection to an institution that does have a credible plan for being around for the foreseeable future should be a huge positive.
This is not a new issue. It wasn't a new issue over 800 years ago when multiple copies were made of the Magna Carta and deposited with different institutions. Four of those copies survive today. Handwritten copies on the finest sheepskin parchment are very different things to digital copies with an institution that has multiple servers in multiple locations, and an endowment that should be able to fund migrating that information to whatever the internet becomes in future centuries. But the principle is a good one, and a role that I think the WMF could usefully step into.
If the endowment has grown to the point where the WMF could now announce that it can be confident of financing Wikimedia Commons and WikiSource for the foreseeable future, that doesn't mean that one penny need be tapped from that endowment while other fundraising is healthy. A guarantee can be issued on the understanding that it is unlikely to need to be redeemed for some years. Hopefully in those years the endowment could grow to the point where the guarantee could be extended to other projects such as Wikidata, WikiVoyage, Wiktionary and Wikipedia. But there is a case for prioritising Wikimedia Commons and WikiSource for such a guarantee, it would open more doors in the cultural sector and attract uploads of materials that could be used to improve Wikipedia and other projects..
I suspect that the endowment is already big enough to issue such a commitment, if not, at the least the WMF should be able to set a target for how big the endowment needs to be for this to be possible.
As for the more topical question of current fundraising and fundraising for the endowment, I for one would be happy with a compromise whereby in future donations would only be added to the endowment if they were specifically given for the endowment, and each years fundraising would stop when it had raised enough money to cover the following year's budgeted expenditure.
Regards
WereSpielChequers