competitive with 25 players per team kekw
I'm not saying tournament matches should be 25v25, but 25v25 makes for a good jump-in/jump-out public server to enjoy and practice the fighting mechanics, which can serve well for the competitive scene. Even with some circle-running, there's a lot less downtime, since there's no queueing or other waiting periods, and in my experience there are far fewer matches that end up being a stressful waste of time due to unbalanced teams or uncooperative teammates. There were top players (difference-makers) who would bother to switch teams themselves on IG for the sake of balance (also, worse players could switch to the better team), and even if the other team was OP, it never felt nearly as dreadful as on MM.
Continuing to play scrims again and again can be overkill in terms of learning maps and strategy, and officials can end up different to anything you might have over-prepared for anyway. I feel like 25v25 public battle is more likely to bring about scenarios that practice your improvisational skills more, which seem very important for competitive. I never noticed a large fight playing out in the spawn area of Fort of Honour in WBMM, yet in the WNL8's Grand Final, that's exactly what happened, away from the graveyard. A lot of the most regular players of WBMM didn't even outplay others who were much less regular on WBMM in the end. The best players in the game played on IG_Battlegrounds a heck of a lot. Peter/Riruro/Morpheus was on it all the time when he was at his best. Playing 8v8 (or 6v6) is obviously useful up to a point, but I remember Napoleon saying that playing on IG (25v25) was often more useful than playing scrims, after his team Freelancers had won their second major tournament.
In the last couple of years (when the scene was dying out) WBMM was better at eliminating trolls, and it was better at getting people playing - waiting on a website to fill, rather than waiting in a server to hopefully seed it. But now the scene is different again. If combat gets fixed (and maybe even if it isn't) public battle servers have the potential to be populated easily again.
A lot of people just don't like MM. There's an inherent lack of cohesion among random teammates (even with a morale system), which can be really annoying, considering the greater importance of teamwork. Going forward with the team on IG was a lot simpler and intuitive, so everyone did it, except for a few talented players who could lone wolf - another thing that was hugely enjoyable and more permissible on IG. A lot of people also don't like being ranked in MM. Smurfing is a huge thing in all of gaming. The sweaty atmosphere (with teammates breathing down your neck) can burn you out a lot too, making you want to play less or give up on the game.
Even if there's good reason to have Skirmish MM, and if it turns out well eventually, clearly enough people still want Warband-style battle servers in Bannerlord. I think it's more important to have.