Crossbows and Bows serve different purposes both in game and real life so it's a shame they're just different notches on a linear upgrade tree. It would be nice for crossbows to deal more damage given their accuracy and bows to maintain their ability to hit enemies without line of sight at a reduced production and damage cost. Players in more uneven terrain will benefit from bows while flatland players will benefit from crossbows. 


A bit unrelated but it would be an interesting unique effect if Furnace Steel increased the production cost of these units while also increasing their damage, helping stress its importance in history.