Yes, but what if we don't like the unit stats, the scale, the unit abilities, weapon differences, how combat speed works, how spells are cast, etc. Can stuff like that be changed?
I would say most of those can still be changed, yes. By changed I mean tweaked in values and such, without needing large chunks of new code. But the overarching system of "Your turn, do stuff, hit end turn, they do stuff" isn't going to change.
I'm pretty sure we are stuck with attack, defense, hp and combat speed describing our units which is pretty limiting for instance. I'm sure there are many interesting ideas we could have come up with if we were allowed to play with it.
We are stuck with it, and there have been interesting ideas, but we're not stuck with it because tactical battles are coming late. We're stuck with it because (as Frogboy posted), they can only do so much with stats when they potentially have to simulate very many battles between turns. Every single interesting RPG-like combat mechanic that gets added can easily snowball out of control if it has to simulate 50 battles of 1000 units vs 1000 units and each individual unit has a list of 5 different combat modifiers that it has to calculate for each combat round. So on, so forth. At the end of the day, they do have to make the game run on as many machines as possible, and there's always tradeoff between system resources and this kind of advanced stuff.