But the turbo's are as off the shelf as any DMU, but XC can't (and others) can't extend them. It may be more of a problem with DMUs but from what I know you can't put a 172 trailer in a 170. Happy to be corrected and if you can maybe TPE and XC should be ordering some to make their 2 car 170s upto 4 car. I am sure many services need them and the 3 cars could be moved to other services. Nottingham - Cardiff 4 car and the 3 cars onto stansted. Likewise on TPE 185s back to Cleethorpes and have the 4 car 170s on busy north pennie routes.
Hypothetical and I know it won't happen, but imagine 100 extra centre coaches for 170s being built.
Scotrail run doubled-up 170s and 170s coupled to 158s, TPE run doubled-up 170s, LM run doubled=up 170s and 170s coupled to 153s, XC run doubled-up 170s...
As far as I understand, every TOC with 170s has them coupled to other units for a good portion of the day, with the exception of NXEA
So, if a hundred coaches were built, it'd allow several 170s to be extended to 3/4/5 coaches, which would free up a couple of dozen units that could be used elsewhere.
In fact, a five car 170 might be okay for some other XC services, albeit pathed to 100mph.
In essence, if enough are ordered to make the unit-cost reasonable then you'd end up with sufficient coaches to spare enough DMUs for a decent cascade
Otherwise the only likelyhood of increased length will be when the "central belt" electrification is completed in Scotland and FSR won't need so many 170s (of course when that happens, FSR will want to keep the 170s on secondary routes instead)