It's not just about archers. It's that Javelins and Hand Axes are fucking broken.
Weapons and skills worked out fine in FE5, although what IS would actually do is entirely irrelevant to this discussion, since as evidenced by the games they've attempted to make without Kaga, they aren't going to do jack shit. And a weapon with 30-40 uses isn't going to last long if it's the sole reason you use a unit.
As for crossbows, I have to say that I'm again drawn to prefer the Berwick Saga take on it: Have separate types of arrows that can be used, primarily scaling Mt in exchange for cost and availability, allowing crossbows to increase their power as the game goes on without needing to use Str.