I think it actually is more helpful to the business to get a loan instead of a donation. With a huge donation you can do whatever you want with it and may overinvest or spend wastefully, until the cash reserves run out. A loan means that the business has X-many years to be self-sufficient. So I think it helps the business's culture as it's developing.
This is like the awful paternalistic version of my thought. Ha. My guess is that there's a implied agreement, "keep to your promises about how Signal will be run and developed and eventually the loan will be forgiven altogether; try to burn me on those promises, like Zuck did, and that'll be a $100 million gamble." And sure, with inflation, the longer they're around, the less daunting the figure will be, but that may be the point, or the loan could be refreshed with more money that just keeps the prospect of repayment a perpetually serious prospect (it was doubled once already).
Oh, I like that explanation better actually. They very well may have an agreement like that where he'll forgive the loan as long as he's still happy with how they're running it
