Never is a long time. Non-profits can become for-profits. Non-profits can also spin off of their profitable components as for-profits -- just like the Mozilla Corporation is a for-profit owned by the non-profit Mozilla Foundation. Non-profits can sell ads and sell your data.
And, of course, non-profits can get money hungry just like any other organization. Look at how ICANN has found ways to vacuum up billions of dollars from the internet.
They also may not technically have any investors, but they have a de facto investor in the form a $100m loan from WhatsApp founder Brian Acton who is also on the board. He could have made it a gift, but did not -- which implies he wants the money back at some point. Signal will need a lot of donations to pay that money back -- or they could decide to sell ads or data.
All that said, I'm happy Signal is doing well, and applaud their efforts. I hope they succeed.