Exhibit A: The Signal Foundation's tax reports[0]
Exhibit B: The fact that WhatsApp had a very small team and rather low costs, at least prior to its acquisition by Facebook. (I can't find any numbers right now.)
[0]: https://projects.propublica.org/nonprofits/organizations/816...
Telegram has close to 500 million active users each month. So of course Signal is not using as much money. The same Wired article mentions that Signal recently had gone from 3 to 20 full time employees, that adds a lot of cost as well.
My point is that I don't think Telegram have spent lavishly or focused on big profits, so it's unreasonable to assume that Signal will be able to do what Telegram does for much less money, so they will also need a new monetization eventually.
https://www.wired.com/story/signal-encrypted-messaging-featu...
[0] https://t.me/durov/142