For my part I dont want to buy it on EGS, because I prefer having everything under one roof. I can't do this for every case, but try to do so. Also, atleast for teh beginning - later than that I dont know - the EGS was supposed to be not safe and lacking functions. Until now I never was interested in any of their exclusive games, so I never read more into it.
(for the time when its coming hopefully on Steam), I'm most excited about the Data fights, where I managed to not spoiling myself on any of these (exceptions being the music and the Secret Boss).
As for mods... I guess modders could try to put the Data versions into the graveyard sequence, but since I dont know what the Data fights are made of (except being difficult), I don't know if these upgraded fights would be possible both technical wise and gameplaywise.
Their exclusivity deal won’t last forever I’m sure. At some point we’ll be having this same hoopla when they announce it on Steam in 1-2 years. It’s fishy to me that they announced this port for EGS to release in March during the 19th year of KH. Just sounds like lead up to a Steam announcement for the 20th anniversary.