If Let's Encrypt believes that's the best way to spend their money, they'll do that. Do you think you've thought more about their sustainability than they have?
I'm not talking about how Let's Encrypt spends it's money, I'm talking about how I spend mine :)
Assuming Let's Encrypt knows more about its sustainability than you do, and given that they can also invest their money in index funds if they believe it's the right approach, isn't giving the money to them outright just better?