If you need to pass a barrier that scans your ticket when you enter, and pass a barrier that scans your ticket when you exit, why do we need these staff?
Because there are many, many stations without barriers (and installing them would cost money and require staff to monitor them). On my local line the only barriered station is Glasgow Central, but there is much intermediate station travel to places such as Mount Florida and Queens Park.
Plus even where the stations are barriered big savings can be made by only having a ticket to work the barrier at each end. I wish to fare evade on a journey from Aberdeen to Edinburgh. The single ticket is £41.40. However I can reduce that to £7.40 by buying a single from Aberdeen to Portlethen and Haymarket to Edinburgh. Now the chances are on a journey of that length the Scotrail conductor would probably do a revenue check and would catch me out at some point however the point is that barriers aren't the be all and end all of revenue protection.
Conversely, how is an RPO without any authority of arrest supposed to ever do anything about people who are determined not to pay?
I'm not entirely convinced that they have the power of arrest in England and Wales (I don't think PACE require an arrest?) but even if they do we seem to manage quite well without constantly arresting people (see the Disputes & Prosecutions section)! Railway staff do have the power to remove people from the railway who are in breach of the Railway Byelaws using "reasonable force" but I've rarely if ever heard of it being used.
But the reality is that there is a hardcore subset who do refuse to pay and they exist both in England, Wales and Scotland as well. Yet that doesn't mean we throw the towel in and don't bother without any revenue enforcement at all. Typically what I've seen happen down here is that the train stops at the next station and the person is invited to leave the train or wait for the police to attend. From the cases I've seen personally most then pay-up and handful leave the train (none have yet opted to wait for the police, though a few said they would).
Unless you consider revenue protection to be window dressing throughout England and Wales I don't see how it will be in Scotland.
The solution to that is revenue blocks at adjacent stations, so that nobody would be able to enter, in this example Bellgrove, without a ticket.
Something which is commonly done (at least before the wider roll out of TVMs and Penalty Fare schemes) elsewhere. It was not uncommon for there to be a total block at Outwood (first station outside of Leeds) meaning anyone presenting at the barrier asking to buy a ticket from Outwood would be met by "May I ask how you boarded a train at Outwood when my colleagues were there ensuring everyone had a ticket before boarding this morning?" and I'm sure the reader can imagine how that would end...
None of what ScotRail is proposing here is anything other than things which have been done down in England and Wales for donkey's years! The incredulity shouldn't be aimed at ScotRail recruiting RPIs now but more that they've not done so before!! Just because they cannot easily prosecute fare evasion doesn't mean they can't still make a difference...