Respectfully, I disagree with this. Just look at CN. They did something fairly remarkable a few years ago: they purchased more than 75 Dash 8-40Cs (former UP and CNW units), and they rebuilt them with new cooling systems and some other life-extending upgrades. They also purchased approximately 70 Dash 8-40CWs (former BNSF/ATSF) and have rebuilt those as well. Their total Dash 8 roster outnumbers their total SD50-series and SD60-series roster.
That's true - but CN also purchased almost 100 former EMD/Oakway SD60's at the same time. I also would be wrong if I had been referring to the entire SD60 and Dash-8 series, but I was referring specifically to the SD50 vs. C36-7/C39-8 and SD60 vs. Dash 8-40C, not the SD60M/I/F or the Dash 8-40CW. Among original owners of the Dash 8-40C, all CSX, NS, ex-CR and ex-CNW units have been retired/sold/scrapped, which leaves only UP units (from what I can tell, half to 2/3 still on the UP roster). Among original owners of the SD60, most ex-CR, KCS, NS and UP units are still in service on their original/merged roads or subsidiaries, with CNW, Oakway and SOO units - and apparently, now CSX units - retired/sold. (*Edit: I can see how my wording in my last post about Dash 8-40C's "disappearing" is ambiguous - I was thinking of original owners and those that have been scrapped, not so much about those that have been resold)
As if that wasn't enough, one of the most anti-GE railroads in America has now embraced GE with the equivalent of a bear hug: Pan Am purchased 24 Dash 8-40Cs and a smaller but significant number of Dash 8-40Bs. These will be replacing, not supplementing, many of PAR's EMDs. And this is a railroad whose former General Manager of Locomotives once told me that they would never, ever, EVER go back to GE. Well, they did go back, and in a big way. And GE is keeping them happy by supplying them with new and rebuilt spare parts at affordable prices, as well as a full suite of so-called "digital solutions" to help the railroad run more efficiently. GE even used Pan Am as the subject of a recent case study. Other regionals and shortlines are watching very carefully and some are following PAR's lead.
It's an interesting case, and I'll be curious to see if other short lines follow the same lead. Many of Pan Am's older EMD's seemed tired and I can imagine Dash-8's are an upgrade. But there again, right next to Pan Am is the CMQ/former MMA, which went from an almost all-GE road to mostly EMD - replacing Dash-8's on local freights with GP35u's and GP38-3's that (although rebuilt) are 20 years older.
The thing with bringing up specific railroads is that you can find an anecdote to favour just about any model. I think the only way to gauge the success of a model is to look at the entirety of its production. I like GE units and I think the Dash 8-40C has had a very successful run - but as a whole, the age/in-service numbers still don't match the SD60. However, I wouldn't be surprised if the Dash-9 series approaches parity with the SD70 series.