It would completely break the tactical aspect balance between gamespeeds, this is a turn based game and it is not trivial to scale how many plots a unit may move a turn without changing the tactical aspect of war one bit.No it wouldn't? That has never been the reason for different game speeds for me, and I expect most people. Game speed determines how granular the game experience is *in general*, and how long you play the same game for, unit speed being fixed is an artifact how how it is implemented. When making the game Firaxis had no choice but to keep unit movement the same, it is just a by product of the engine design not supporting fractional moves, not the purpose. To illustrate further: if this game *did* have fractional movement, do you think Firaxis would not have added different game speeds? The answer is obviously that they would have, if only because some people want to play a game that only takes a day, others want to play the same game for weeks.
With scaling:
Either a spearman should be able to move 20 plots per turn on blitz gamespeed to allow it 1 plot per turn on eternity; or eternity should only allow a spearman to move 1 plot per 20 turn to allow 1 plot per turn on blitz. This would be unrouted flatlands without features with movment penalties.
How could we preserve the balance of the tactical aspect between gamespeeds in a turn based game with such a scaling? No there's a very good reason not to mess with how far each unit can move as it is an essential part of the game that a unit can move exactly this and that far each turn regardless of what speed players want the game to progress at.
The whole point of the gamespeed options is to be able to do more with your units during the same amount of change in game situation (buildings built, units trained, techs discovered). So that you may move a your units 10 rounds more before a tech is discovered or another unit is trained.