Historical plausibility and gameplay need to go hand in hand and there is a limited space this is possible to do so. If by "realistic tactics" you mean that the vast majority of battles play only like how you read in history books, then i suggest you just try a few online battles against decent mp players and then we can have this discussion as now we don't talk about the same thing. Once you do play a little against decent opponents other than the AI, you'll also stop putting the word balance in quote marks, and you'll naturally and intuitively come to see why it is important to have a good gameplay.

Just compare cavalry charges in MTW and cavalry charges in M2TW. In MTW charges aren't strong enough to make other army components obsolete you have to use all army components (missiles, melee, cav) to win battles while in M2 you can play with cavalry armies nearly alone throughout because of how strong the charge and mounted missiles are. Then all the historian-minded people like you who want "realistic" tactics play for 6 months before moving to the next "realistic tactics" TW game because "the gameplay wears off"; what a suprise.

Its boring of course precisely because in order to make the tactics "realistic" to a history buff level the dev. cookie cutters combat, army style and potential tactics via mechanics and stats for only the "realistic" tactics to work. That's how CA models TW games from RTW onwards anyway ie with an SP mentality/need in mind, but its older games that were more well thought out from a number of perspectives can still give challenge in SP too other than in mp while the newer ones don't. Try playing RTW or M2TW for a long time and then go back to MTW and see what happens. You get trashed for a while before you adapt, and not because of the engine but because the games are so skweed to certain units and certain tatics to be "realistic" that you can win without having to combine your army components to a high degree.

In terms of realism, every unit would have some pikes up front, some archers behind, some heavy infantry. In other words it would be a unit that in TW terms it can fulfill more than one role. This means that tactics become no tatctics as you have one unit that can do all and there is no need to coordinate your units. Gameplay loses the match and coordination elements along with all the maneuvers that go with those. Try to play this "realistic" tactic and see how much fun the game can be.

Anyway, as i said, if you play sp only i rest my case.