There are really two questions here. First, when does it become unreasonable to keep repairing 844? Second, and not necessarily related, when does UP decide it's not worth it any longer?

For the first question: 844 worked for 15 years in regular service. That's every day work. Since 1960, it's been used for excursions and special trains. That's NOT every day work; it's working, say, one month a year. So 844 has been suffering wear and tear at, say, 1/12th of the "normal" rate, since 1960. (I know, it's a crude estimate, but I think the actual rate is less than that.) That means 844 has experienced 15 years of use pre-1960, and then 57/12 = about 4 years of wear and tear since then.

Steam locomotives in ordinary use regularly lasted 30 years. That leaves 844 11 year of useful life left. At 1/12 of a "year" per calendar year, that leaves it 132 calendar years to go.

For comparison, look at the D&S engines. They were built, I think, in 1927. They work considerably more than 1 month a year, and they're still going strong.

Now, cynics will note that steam engines lasted that long WITH GOOD MAINTENANCE, and that 844 hasn't been getting that in the last few years. That's a discussion to which I have nothing useful to contribute, so I will ignore the topic.

On to the second question: UP will ditch the steam program when they decide that costs outweigh the benefits of the PR. I think the first fatality caused by the steam program will probably kill it, and maybe the first injury that it causes.