Quite possibly, but start considering the economies of scale, how many such routes are there, and how many units would you need to run all of these national routes...
1) Say that growth sits at a steady 7%/annum on these routes, this would mean that within 10 years demand has doubled, hearby filling the unit twice the size, within 1/3rd of the lifetime of that unit.
2) Lets assume that growth is at a little over 3.5% for any further calculations, I believe this is a reasonable assumption.
Lets say that a 153 runs on a route with a current seated load factor of 50%, on the pessimistic side that means that it will have 36 passengers, on avarage, using the unit. Lets also assume a standard deviation on this figure of 18 passengers, meaning that only 2.5% of the time, some passengers need to stand. (Assuming a Gaussian Distribution)
Now if I where a passenger on this route, I'd be pretty happy with that level of service, but now lets change this for a 1 car 172 style unit, this would have less seats etc. due to DDA requirements so lets say, 54 seats.
Now the probability (without any demand changes) of passengers needing to stand for their journey changes to become 16% of the time. (you may have noticed I'm keeping the numbers easy, mathematicians among you).
Now lets factor some growth into this notional set of figures.
By the time that you end up with half of the services being run with passengers needing to stand, it would require 18 more passengers, on average to travel per service. That would be a 33% increase in demand. Within 10 years (at a convenient level of growth, now an even more conservative estimate of only 3%/annum).
So, within 10 years of this notional change in unit, at 3% growth, 50% of the time, passengers would need to stand for their journey.
Now I'm sure we can all appreciate that if you had to stand for half of your journey each time you'd be pretty annoyed at this and start looking for alternate modes of transport.
So if we don't want to stifle growth, then a 50 seat unit to replace an on average half full 153 will only last for 10 years before hitting a point where it will effect demand on the route adversely.
PS to point 1: If you don't know why 7%pa (Or to be spesific 7.1773462536293164213006325023342%pa growth in 10 years means double demand, read a book...)
Now if we look at replacing a 153 with a 2 car 172. The LM version has 116 seats so lets pick a number close to that.
Now, assuming the same initial requirements, these 36 passengers, would be divided between lets say, 108 seats, some proper luggage space is provided.
That means that if growth remains the same for the previous example, 3%/annum. In 10 years, passengers will only need to stand on 0.15% of journeys.
With the higher levels of predicted growth at 7%/annum, passengers in 10 years will only need to stand for 2.5% of journeys.
If you stick half way between, (5.2% growth) in 30 years at the notional end of life of the stock, passengers will be standing on 16% of journies. Likely to be the peak hour services.
Now factor in economies of scale, the additional cost of developing and maintaining this fleet of 153 look a like units at most approximately 50, compared to adding them on to an order of 172 units. In addition to the fact that in 20 years they may be superfluous to use anyway.
Just think about some of the numbers there...
Not to mention that a shiney new train thats twice as big as the old one will release (as said above by ainsworth74) will reduce a lot of supressed demand on the routes, so growth could go above 10%.