I think we might be overstating both Ranger's history as a reliever and his value as a starter. He hasn't pitched out of the bullpen regularly since 2021 and was moved back to starter in August of that same year. His subsequent relief appearances have been less because he's magically flexible or experienced as just, the normal thing good starting pitchers do in the post-season (it's only happened twice over three years, both in 2022 and never since).
At the same time he was neither durable enough nor impressive enough of as a starter to keep the Phillies from paying Walker like he was going to replace Eflin as the #3 in 2023. And Walker did produce more WAR than Ranger that season, while they both had ERAs over 4.0. That was the year Ranger started on the IL thought he was plainly the better pitcher in the second half.
And then came 2024 and he was an all-star/Cy Young candidate for the first half, and it all went bad. So yeah, it's possible his value as a starter isn't that different than his value as a reliever. But clearly his goal this season was to rebuild the former.
I think it's moot for now, and if it really turns out the Phillies have so much healthy pitching depth that they consider it, the value play is probably still to trade Ranger. Because an above average SP is worth more than an above average RP every time.