not sure if poster is serious ?
maybe the poster needs stringing up on the none existant string
There is the small matter of the track from York-Liverpool not being electrified apart from small sections in Leeds, Manchester and Liverpool. Thus 350's would not be much use.
They can't be used on those routes because the majority of the Liverpool-Newcastle route hasn't been electrified.
No wires on the York to Leeds stretch nor between Leeds and Manchester Victoria. Fortunately the Mk5s are due in the next few years so not long to wait now
I'm assuming that the poster meant "Why can't 350s be ordered for TPE once the wires are up, rather than ignoring the fact that those wires aren't yet in situ...
To which the answer is that by then (indeed by tomorrow!) the 350s will be old technology. Siemens would not want to resume production of an old model, and may not be able to do so easily or cheaply. There's also the argument that 350s, like the 185s, are not ideal for longer distance passengers. The stock that HAS been ordered will strike more of a compromise between the needs of short-distance commuters (Huddersfield to Leeds/Manchester, Preston/Liverpool to Manchester) and the needs of longer-distance travellers (Newcastle to Leeds, Scotland to Manchester).
The main criticism of 185s is the lack of capacity, and mistakes by the DfT under Labour are largely responsible for that. The transitional period between now and electrification completion will see 185s continue to run between Hull and Leeds and Manchester, and picking up intermediate calls between Leeds, Huddersfield and Manchester. It is this which causes me most anxiety, as peak stoppers are crowded on these routes as are the current TPE services which serve Dewsbury and Stalybridge. Squeezing pax from these two services onto a single 185 (planned extensions to intermediate stops won't allow 2x185) won't be easy or pleasant even if unit deselect is used!
What will happen once the Manchester to York wires are live is not certain. Hull wires are uncertain, as is Middlesbrough. Scarborough hasn't even been subject to a wibbly thread on here as far as I can remember. Bi-mode sets could be used, though continuing with 185s may be the least-worst option even though diesels under wires makes my blood boil. There are many "If onlys" with regard to 185s - If only they were longer/more plentiful, if only they had end gangways, if only they'd ordered more once the growth became apparent... (for the record, overcrowding seemed an issue to me even off-peak long before the 185s came on the scene). With wires now coming eventually, another one is "if only they'd been supplied with electric transmission for possible conversion to bi-mode!".