Thought you were a fan of "S-bahns", Neil.
Let's face it, the biggest reason Merseyrail is such a success is the high frequency, uniform all-stops service delivered by one company overseen by the local transit authority.
I wasn't insinuating anything about the reliability of the specific rolling stock, although I'd argue that an old District line unit converted to DMU and running on battery on the Merseyrail network has a somewhat experimental aspect to it (no doubt a big part of the trainspottery attraction to the idea). More, I was talking of a single, bespoke hourly service coming in and reversing out of a station that current sees 14tph going into and out of it.
And upsetting the uniform experience of a metro system is more than just an aesthetic consideration. For instance: is it possible to cater for level boarding at stations with both types of unit? Presumably, platform edge doors or gates, not an immediate prospect admittedly, would have to be ruled out or even just painting where the 4 & 8 car trains stop and where their doors will be - things Merseyrail could consider, once the new stock finally takes over, that much of the rest of the rail network cannot. Speaking of doors, the Class 230 isn't exactly known for fast closing doors and quick dwell times. Not going to exactly help at Hamilton Square at busy times.
And comparing anywhere to London isn't especially helpful. Of course Liverpool is small if using that yardstick. But if you want Lime Street, the Hospital and the area around the university, James Street is a bloody good walk and an uphill one. If I was on a Borderlands train, I wouldn't think twice about changing at Birkenhead North for 8tph into Lime Street or Central.
Regardless, this is one of the seldom occasions where the reality machine is on my side - it isn't going to happen.