Talking from an interlude player perspective:
I'd say it's mostly the "fear of the unkown". Also some of the advantages you've listed in using client base would probably turn interlude players away as it is no longer an interlude (new olympiad for example).
Interlude files + customs from higher chronicles -> people know what they get plus some nice customs
classic files + certain interlude features -> people would be unwilling to try it imo (questionable files + too many new things that it's no longer an interlude)
PS: About oly/pvp in interlude, there could be LOTS of improvements, especially in java interlude, yet im shocked to see that barely anybody has bothered with them.