Does it all not come down to money, I would guess the 158s are cheaper to lease than 170s. Southern seem to need a few and 158s will require crew and maintenance costs whereas 170s just need the coupling change. There have also been many posts on this forum that have simply said Scotland will not give up any DMUs, clearly whoever runs Scotrail in the future will have to balance the books.
Sorry if anyone thought my earlier post was a "rant" not intended as so.
Your view is actually quite understandable as Scotland does do significantly better than many parts of England in terms of stock provision, something enviable and the frustration felt in many parts of England comes from this. I often travel from Stirling to Newcastle via Glasgow, so a 170 to start with (very nice, free wifi, recently refurbished), then a 185, 221 or 390 down the WCML for a 142 along the Tyne valley, so a notable step change in journey quality!
As has been mentioned, TFS is flagging up the 170s as their prime train, hence I support the notion that FSR will hang on to these units as opposed to 156s and 158s. It does make sense, but obviously there will be a cost. The 156 fleet will start to diminish once wiring starts to East Kilbride and Barrhead, then Shotts with both the E&G scheme and up to Stirling/Alloa displacing a fair number of 158s/170s, which I am confident will be retained by FSR to allow 156s to be displaced from the GSW and WHL, then cascaded away south. I know clearance works will be required on the GSW and WHL for the units, but I've already predicted that the FSR diesel fleet by 2020 will consist of possibly just the 25 Inverness 158s and current 170 fleet, possibly supported by some 221s ex-WC, which TFS have been suggesting as desirable for the Highland mainline.
As for giving up units, the 2014 schemes (Whifflet/Cumbernauld) only displace 6 units, sufficient to tide FSR over during the Commonwelth games peak and then available for the Borders line in 2015. Moving on to 2016, when the E&G goes live, this should allow 10+ units to be displaced, then 2018-2020 will see a glut when the remainder of the Central belt and Glasgow south west should go live. So, yes FSR will give up considerable numbers of DMUs, but only when the wiring schemes progress and I would suggest most of these units will head to Northern, directly or indirectly as Pacer displacers.