But if there's money for dozens of brand new DMU carriages for the North, then it's surely daft to replace 185s but leave Sprinters in service?
I agree to a degree (although, obviously we're not going to see dozens of brand new DMU carriages, so none of this will happen)
However, we have to look at the routes that 185s could be cascaded onto. Some people are just looking at the age of trains and wanting to replace the oldest ones, but Northern has a lot of services that require nothing more than a 75mph DMU with reasonable acceleration (since the stopping nature means that station dwells are more important than top speeds). 195s can fill that role, but there aren't anywhere near enough 195s to replace 150s (and the OP seems to want to focus any additional 150s on longer distance stuff like Manchester - Cleethorpes)
There may well be 175s and 185s on the second hand market but will end door 175s be able to cope on services pathed for 150s? There's not going to be much need for 100mph operation on Northern services (in fact, about the only Northern services that could run at 100mph are the ECS between Heaton and Yorkshire, to rotate stock that is maintained on Tyneside but operates a large chunk of it's life in God's Country).
Personally, I find 150s pretty horrible - very basic BR stock, the "toaster rack" 3+2 seats have very low bases - I'd much rather have a 175 in theory, but would that 175 cope with the dwell times at some key stations?
185s are better than 150s too, but do you need the incredible thirsty/ heavy 100mph DMUs on many Northerns services? The 185s were designed to do one particular route, and would have done it well if the Government had allowed the fourth carriages, but where are they going to go on Northern's current network? The S&C is often discussed, but that's only about three units.
Look at how 170s struggled when trying to run stoppers like the Harrogate services - your top speed looks nice but if you can't get up to 30mph/ 50mph in reasonable time then you're going to struggle.
There's also the problem that Northern struggle with so many different DMU types (150/155/156/158/170/ 195 - including different subclasses/ types of some of those trains). Introducing a dribble of 175/185s into the fleet is going to complicate things further.