It's not. There is one reason and one alone - new was cheaper. Originally some of the 730s were to have been 3+2 anyway.
I was meaning why they sent the /2s back to Porterbrook and didn't return other subclasses. There's been LNR managers promoting the benefits of the refurbished 350s to passengers using the Liverpool route. One of them being 8 car formations being possible. The other being they've updated the interiors to provide more comfortable seating.
I've previously noticed the /4s have had the 2+1 seating in the former first class area, with the exact same seating that's in that area on the /1s. Not an improvement, but at least it's consistent between sets.
Any comment about the 4 planned 4-car trains? I guess you could try to justify it by pointing out that they almost all leave Manchester after 6pm, but I bet the 1803 to Brum and the 1825 to Bournemouth are cosy, and even the 2025 to Reading could be crush-loaded.
Right why are you asking me to justify something that doesn't need any justification, when you've have twice also said you want to move on as it's off topic?
At no point have I sad 4 or 5 carriage trains are adequate. The suggestion was if 350s replaced Voyagers. 350 carriages are shorter than Voyager carriages. They would need refitting if a refreshment trolley was provided, and if First Class seating was to be provided. Therefore, to not reduce capacity you would need 2 x 350s to replace a 4 or 5 car Voyager, or 3 x 350s to replace a 8, 9 or 10 car Voyager working.
The morning peak arrivals at Piccadilly around 09:00 (Northern, XC, Avanti and TfW) seem to work quite well overall. The stopper from Stoke is followed by an Avanti service that calls at Stoke and Macclesfield. So if you're boarding at one of those two it makes sense to wait for the longer Avanti train, that'll give you a lie in. The pair of XC Voyagers go via Crewe, calling at Wilmslow. That arrives after the stopper from Alderley Edge, and just ahead of the service from Cardiff (which has significantly less capacity).
It doesn't work so well in the evening peak though, with the 17:30 departure towards Cardiff being a single 197.