That's probably not a sustainable situation.
IMHO funds received by well-run non-profits will be banked, not spent, therefore they yield ongoing returns which are used to meet costs and sustain the organization. The fund origin is immaterial.
Meanwhile, the whole idea of iNaturalist has evolved around voluntary reporting, community involvement, and open data, and I think some of that needs to stay. They can't turn fully commercial.
If in fact what you said is true about the sources of funding, it would then seem that the US taxpayers (the relevant party here) are footing a large part of the bill from direct and indirect subsidies. I feel that it can be reasonably argued that a non-profit organization that is benefiting from significant public subsidy should make their model available for public use.