This is actually one of the reasons I like the war history. It lets casual alliances be matched more often with casual and hard core with hard core. What you call underachieving are just alliances that have different life priorities in many cases. The hard part is often finding an alliance with the same life priorities that you have.
As an alliance leader of a team that frequently has people become inactive, this is usually not an issue. If a player is participating but doesn’t use any war flags for like 3-4 consecutive wars, they’re automatically removed from war participation. I had one alliance member who complained about that frequently because they’d go inactive for like a few weeks then come back.
Yeah it sucks for that particular war, especially if the player has a high score (as mine did), but it self resolves after a few wars.
Should be no big deal if the inactive alliance member does not participate in AW?
Of opted out, either by choice or automatically through inactivity no impact. If opted in, it can have varying degrees of impact depending on size of the alliance, if the member has a powerful team, and the relationship of that power to the rest of the alliance. But if opted in its always a drag on the alliance