If they had a lump sum donation of $100M, they could probably safely expect a return of 5% per year on average (many years could be worse).

As long as their costs are less than $5M per year, they can effectively run forever, barring any long-lasting financial depressions (the likes of which we've never seen), or severe financial mismanagement.

This is how many museums and universities operate, and what the above commenter was referring to. :)

Okay but museums and universities also rely heavily on ongoing donations. A 5% annual yield is far from a guarantee. Not to mention that running a tech company is very different than running a museum or local symphony. A tech company has to attract and retain top talent that is very expensive and costs scale with adoption.

Ok, then I'll speak specifically about my local institution. My local art museum gets zero funding from taxes, but runs on its endowment and whatever revenue it brings in from admission, membership, events, food and beverage sales, and merchandise sales. Are they unsustainable because of that?

How is this applicable to what I wrote?

Here is the relevant part again:

> because its revenue from goods and services is less money than it spends

You're saying that a company can't be sustainable if its spending exceeds revenue, but there are plenty of institutions that do exactly that and are based on the same premise as what we know about Telegram's funding. A single (or multiple) large donor(s) can potentially donate enough money to make a thing sustainable without relying on much if any revenue. There are examples of that in the real world.

How many of them did you gave your private secrets?