I think we’re speaking at cross purposes then because I was mostly speaking about the campaign. If Wesnoth was designed only for one-off battles and units were meant to be sacrificed like chess pieces in singular pursuit of a victory then a campaign is wholly inappropriate.

The original point of comparison here is Heroes of Might and Magic which does not have this problem since the game is designed as a campaign with many battles and the leveling up aspect only applies to heroes which are essentially generals that do not directly participate in battle.