I find it hard to believe they are only viable for the exact amount of tunnel we're digging for this extension. Of course they need maintenance, but worthless afterwards? That's transit planning for you.
"Hey, we should build a Spadina Subway extension."
"Excellent idea Johnson, lets spend
$58.4 million on the TBMs which we'll scrap afterwards because we'll never dig tunnels again in Toronto."
... Several years later:
"The Spadina extension is coming along nicely, we should be finished tunneling soon. Maybe we could use them for the Eglinton Crosstown we're planning."
"No that won't work as we've decided to use LRT, even though we're burying the majority of the line, and they require larger tunnels."
"Ok guess we'll just put out another multi-million dollar contract for new TBMs, which we'll scrap afterwards because we'll never dig tunnels again in Toronto."