I recall a LOT of people complaining about Alberian Front last year, especially after the first week, due to how long the average game took combined with the valor grind, especially with the volatility of pubs and the fact that the original run also lasted 3 weeks. I'd say the reason they locked out co-op entirely this time around is because they're knocking several birds with one stone here: seeing how receptive the playerbase is to a solo-only version of this mode in order to potentially make future single-player content, making sure wings aren't being spent too much on the re-run if the new event will heavily feature multiplayer, and possibly beta testing for an Event Compendium-version of this event. Since Nintendo owns the IP, FE is probably the easiest to add to the Event Compendium from a licensing PoV compared to MH/Mega Man.
Right now, I'd say it's pretty important for people to consider what the devs should do to make the solo-version more enjoyable and tell them about it through feedback. If defensive battles as a whole just aren't unenjoyable solo like some folks are saying, and it's legitimately not viable to make it compelling in a single-player environment, then I don't think it's in the devs' best interest to dedicate a ton of resources in making more of this mode, when they could spend that time on content that could potentially be enjoyed both solo and co-op. Especially when they've been trying to figure out how to go about endgame single-player content on the level of High Dragons and Agito, having yet another piece of content that basically requires co-op to be fun doesn't seem like the best use of resources.