Seems it'd be far more reasonable to direct the excess funds into an endowment of sorts instead of spending it on other areas.

If they did that they'd already have a big enough endowment to fund all of Wikimedia for life.

Well, Wikimedia does have an Endowment, currently valued at $169 million. See Form 990:

This is in addition to assets of around $300 million held by the Wikimedia Foundation. Form 990:

For expenses breakdown see the most recent audited accounts:

	▲	ymolodtsov 8 hours ago \| parent [-]
		$169 million is not too big for an endowment, especially considering how much they had been raising AND spending in the years prior. Long-term it'd be best for Wikipedia if it was self-funded through this, like Harvard, for instance.