relatively few tunnelling operations of this scale have been attempted
More than you think, I suspect; TBMs are a fairly mature technology. Crossrail is not as impressive as the Channel Tunnel, which is a major feat of alignment as it was tunneled from both directions to meet in the middle.
Wikipedia has a list of long tunnels [1]. And "Tunnel Talk" lists what the world's "mega TBMs" are up to [2] (frustratingly they quote tunnel diameters but not lengths). Seems like these machines are kept very busy, which makes sense given their cost.
I believe that many of them are actually single-use. Some (or perhaps all?) of the Crossrail TBMs are being buried at the end of their tunnel. Apparently the effort of removing them isn't worth their resale value?
That tunnel talk page does mention that several TBMs are being reused though so perhaps this is changing with newer/larger machines.
They operate in an extremely hostile environment, which takes its toll, and a bit like fighter planes there aren't enough built to make a consume-quality product. Some are reused but they'll always go through a major overhaul in between doing so.
If you do not take them out (which would have meant dismantling them and dragging the parts through kilometers of tunnel), they have to be, or they would block the tunnel they dug.
The article mentioned £10 million per machine which, as a complete layman in these things, I found incredibly cheap, even assuming maintenance costs would likely out-weigh the cost of the machines.
OP indicates that the TBMs cost US$15MM each. I'm surprised they're so cheap. Those things are 50m long and full of complicated parts. It seems like the electronics and custom steel parts would cost that much and the actual design, construction, building, and testing the thing would cost several times as much.
At US$15MM, the labor to run them or even just to do maintenance would cost much more than the machines themselves.
Yes it's a niche market, but the markup isn't there because the companies that make TBMs don't actually MAKE much of them. There are probably 100 or more companies that make the parts and the company that says "that'll be $15mm please" doesn't do a lot more than put together the design from the various parts and contract everything to be produced, manufactured, etc.
What they do have that you or I don't is contacts and business relationships with all the various suppliers and the knowledge of the myraid things that you need besides a bit spinny thing at the front, the conveyors that move the stuff out, and the hydraulics that move it forwards. In a very basic sense that's all a TBM is. But of course it probably does at least 20 other things that are absolutely vital to it functioning properly and efficiently.
But if a few senior guys decided that they wanted to work for themselves they could probably take enough supplier relationships and domain knowledge and start up another TBM company.
That's only done when you have a meet between two TBMs coming from opposite directions, and there's no way to get them out. Only for very long tunnels is that necessary. For most TBM jobs, you get the TBM back when it breaks though at the end of the job. After refurbishing, they're good for another job.
Go to TunnelTalk.com for the best deals on used TBMs.[1]
The two ends of Crossrail "met" in a big prebuilt cavern in Farringdon. Still impressive, but not quite on the level of joining up two tunnels under the sea.
More than you think, I suspect; TBMs are a fairly mature technology. Crossrail is not as impressive as the Channel Tunnel, which is a major feat of alignment as it was tunneled from both directions to meet in the middle.