It depends on how much it's played. A Steinway in your home is likely to last a lifetime or more. The Steinway in the recital hall at our school is between 20 and 30 years old (I think), and well past its prime. It's played (hard) probably 4-6 hours a day, and eventually they just wear out. It would make a fine piano for someone's house, but it's no longer a performance-quality instrument. (Our own personal piano is a pre-WWII instrument, a 5'9" Knabe with ivory keys. It's been rebuilt once, and is a little...quirky, and could probably do with another rebuilding. It was also a very good deal for two just-out-of-grad-school musicians.)

Some music schools have contracts with Steinway where they get all-new instruments every year, which are sold back to Steinway at the end of the year who sells them as used instruments. I'd imagine (but have no evidence) that music schools in general are one of the largest consumers of pianos. At our undergrad institution of ~200 majors, we have probably 30-40 pianos in the building. My graduate school probably had upwards of 200, and employed 3 full-time piano technicians to maintain them.

My gf's Steinway is nearly a hundred years old. If they're refurbished every 30-40 years or so and kept in conditions that make sure the frame/case don't warp or crack, they can last indefinitely.

It's relatively trivial to do a minor refurb to replace and realign hammers/felts, rather more expensive to replace the tunings pegs and strings, quite a big deal to do all the above and replace the soundboard too.

Gf's instrument is getting near to needing a complete rebuild, which we're estimating will cost around £6k and take a few months.

I can appreciate the interest in digital instruments, but nothing sounds close to the sonic richness, weight, projection, and detail of the real thing. You certainly can't play classical piano music on a sampled keyboard and have it fill a concert hall the way you can with a Steinway Model D.

The same thing has happened with pipe organs. There, the cost of a physical instrument is much larger—modest instruments are well over six figures, with many instruments reaching $1 million. Digital organs have found a place in many venues—especially those, such as concert halls, that only use them infrequently. However, there is still a thriving community of builders churning out new pipe organs, and a bank of speakers still can't match the power of thousands of individual pipes.

I saw an organ in the Netherlands that dates to the 1700's, and has been played by many legendary composers (including Haydn and Mozart, IIRC). While it's obviously seen continuous maintenance, it's still playable and used for performances today.

Growing up, I had a 150 year old upright (I forgot the make). It had a great sound, but a bad move between homes cracked a major support in it and it would have cost more to restore it than to buy a new one.

My parents finally sold it to a tuner for scrap long after I moved out.

In high school I was an exchange student and had a chance to play on a late 19th century "grand" style piano forte that was still in perfect working order and seemed like it would be good to go for quite a while. It didn't sound quite a nice or bright as a modern piano and had a smaller keyboard, but it worked well enough.

I think the Chinese market has helped suck up some product.