17 months of operating expenses are actually not a lot for a foundation. Especially one whose goal is to preserve something for a long horizon.

Unions exist to combat the monopsony power of corporations. Corporations and unions can exist in constant tension with each other because ultimately both are bound by the market of their product.

I don't think the logic holds up when you're talking about foundations or charities. I'm donating to Wikipedia because I want to advance their cause. If the unions goal is to raid donations and get an increasing share, that could potentially go bad.

Worse, the union can sometimes capture an org and begin to exert control of the mission.

Even if you're very pro-union, there is legitimate reason to be hesitant here.

Non-profit executives are even more capable and better situated to "raid donations" or change the direction of the mission, and can do so a lot easier when there is no organized labor force to push back against it.

On the countrary, nonprofits need unions more than for profits. They exploit their workers more. They have fewer resources and exploit their mission to get more work from their workers.