Yes, we need a rolling order of 172s (or equivilent) to replace ALL stock pre the 158s (and then the 158s too). I've always said this.
I wouldn't say that exactly. 172s with their suburban design are not really suitable replacments for classes 153 to 159. Hopefully electrification (directly or indirectly) will clear most if not all the Pacers off the network so that leaves an order of 75mph 172s (the London Midland version with corridor-fitted-cabs) to replace 150/0s and 150/1s (life-extend the other Sprinters in the hope electrification will proceed fast enough that most won't need diesel replacements). Life-extending the 153s is expected to entail converting them back into 155s so I would suggest an order of 172-based trains laid out like a class 175 (doors located in vestibles at the ends of vehicles, seats with legroom and aligned with windows ext. (but with padding, unlike 175 seats)) but with corridor-fitted-cabs like 158s/159s, 377s and
LM▸ -172s to boost the numbers to compensate. This would be a 90/100mph unit, perhaps including a pantograph car (creating a bi-mode, but with the lower top-speed and 23m vehciles this would be a far more useful one than the 125mph train with 26m vehicles
DaFT» are planning with
IEP▸ , since most (probably all) the 125mph lines will be electrified by 2020).