With all due respect to your well-reasoned and reasonable point, CSX never moved on from full PSR, they just simply pivoted to growth after implementation as part of the plan.
The very basic, essential characteristics that drive the railroad business, like being a capital-intense network business with multiple lines-of-business, is the dog here (and elsewhere) to the tail of a tool, not a magic wand, for growth in PSR.
I don't see much, if any, chance NS coulda, woulda moved for PAR. Since the industry was deregulated they never were able to get the dominant Water Level/West Shore network serving New England. Consequently, the amount of money they would have had to spend from Keag, across PAS and down to Harrisburg and out to Buffalo to try to unseat CSX's market dominance and squeeze out the benefits justifying the move simply looks like a bridge too far. Unlikely even in a scenario where Hunter Harrison applied PSR to NS with CSX playing catch up, including a protected New England ownership exit, and certainly, to Mr. Barlow's point, in light of other competing ways to invest for growth.
Looks to me like CN was more of a contender, probably from the kind of opportunity and threat the sale presented in light of limited other ways for them to grow, even with an even-poorer network. I wouldn't doubt that might have had some part to play in CSX pulling the trigger instead of a neutral non-Class 1 keeping the chessboard pretty much static. And, CP, well they definitely had much bigger fish to fry.
So, did PSR really keep NS from buying ST? Nope. But it did allow CSX to and might allow NS to stay in the long PAS/B&E game. And, it will be interesting to see how the 3 Class 1's try to maintain or improve their position in New England given the STB-conditioned open ex-PAR gateways, neutral operation of B&E including in its relationship to GWI-owned properties, and improved NS intermodal business.
Circling full round, in the impact of a declining coal business once accounting for ~40% of revenue with a sub-50's operating ratio covering up the sins in the carload merchandise business and providing the virtue to give rise to the tougher intermodal business, all within a serious on-going industry struggle with how to grow, it's easy to see why PSR came to the US and spread so quickly, and, why it will remain.