That's absolutely right but boilers aren't the only thing to wear out. With so many moving metal parts, despite an automated lubrication system delivering oil to most of the surfaces even the best designs of steam loco will shake themselves to bits before long. The regular expansion/contraction cycles as it's warmed up for work and cools down after also put a lot of stress on the metal.

Sixty years ago refurbs would be much of the work going on at any large railway works - taking engines apart, stripping them down to the chassis and reassembling almost a new one in their place. The ingenuity today's preservationists demonstrate in doing this without any of the facilities of a plant is truly a thing to behold.

(Source: some of my family restore steam engines. I once tried to drive one and I was terrible.)

I should have been less terse. I only wanted to point out its 'old news' -but I love steam, and whilst I love prosaic diesel shunters as much, I do love steam.

I wish they'd do something a bit more streamlined like "Mallard"