Nobody would invest 10 BTC and not share equity. He doesn't own the cathedral, he owns some shares in the cathedral and so does everyone else who invested.

Large, complex, long-term projects will invariably require equity-investment.

You don't give him 100 BTC, he finds 10 people and they each give him 10 BTC, and they all enjoy the common profits and losses and they all get to use the cathedral.

If it doesn't work out, you are out 10 BTC and you have no cathedral. It happens. Shareholders get wiped out all the time. Maybe he has a heart attack and dies and construction is canceled, so they sell the building materials and you get 2 BTC back.

If it does, then you are still out the 10 BTC, but you earn income in the form of fees for classical concerts, tours, weddings, access to the great views from the church tower, or whatnot, and you have a cathedral.

If you are afraid of losing 10 BTC, just keep your BTC and go to someplace else's cathedral.