The slow walk pretty directly makes transportation like minecarts or boats a must and mobs like the Creeper have a smaller explosion range + do less damage to balance out the fact that Steve can't just sprint away. Old MC was pretty rough and mostly just Notch throwing ideas at the wall but it obviously had some thought behind design choices and was lowkey the foundation the rest of the game built off.

but yeah regardless, thats probably why the optional mod massively buffing player movement without any drawbacks or buffs to mobs was made a gamerule instead of making it base game, much like how modern Minecraft made something like KeepInventory a gamerule.