I wouldn't find it unsatisfying. To me it served it's purpose to tell what happened between Sol/Brian. Their love story, what happened with Sol's brother, and what happened to Brian. It gives context. The author just decided to tell it as a whole season then back/forth flashbacks. It wouldn't be the first time I've read a series where the second book (or a large portion of the story) was a flashback. From what I can tell, most people haven't even enjoyed this flashback (though enjoyed Brian's character to a degree), so I don't think most would be dissatisfied either if the season stayed as just a flashback. If there needs to be a greater purpose, it could be for an emotional build up. Sol wanted Brian back. The relic is destroyed causing the memories to be lost. That would be an emotional devastation to Sol that we would feel best as readers from the season 2 build up of their story (just an example). But that's just my opinion.
However, given that, I would be dissatisfied with picking up this story, enjoying Ian as the MC only for him to be erased or merged with a character that has been dead to me (and the story) since the beginning. Will just have to see what season 3 brings. ╮( ̄▽ ̄)╭
Brian and Ian are SO different idk how the author will manage to write them well in the next season when Ian regains his memories. If we go back to Ian as he was, we'll miss Brian greatly and this whole flashback will feel wasted, but if Ian completely changes it'll be really bad
I hope the author finds some middle ground that doesn't disappoint us too much