As many fear, if the existing or future owners were to run the railroad strictly by numbers (which I never agree with), if you could still haul 90-95% of the passengers at 75-85% of the cost. Assuming you had a nominal 20% profit (expenses at 80%), then with steam for each $100 of revenue you would have $80 in expenses and a $20 profit. If your revenue drop to $90-95 but your expenses dropped faster to $60-70, your "profit" would at worst hold at $20 or at best increase to $35.
What I would propose as a more likely scenario as others have suggested is to use a combination of steam and diesel. They could potentially swap engines on a train so of two trains half their trip would be behind steam. They could run Cascade turns with steam to keep costs down using diesels to Silverton, or vice versa using diesels on Cascade turns so they could be offered at a lower fare due to lower costs. The other option this gives them is able to charge a premium for steam powered trains vs diesel. And there are certain special trains that would be economically viable with diesel vs a struggle with steam.
I don't think steam will be eliminated, but there will potentially be a new operating and business plan in the near future.