Everyone who wants to know why the Coalition Ships are being replaced is because the current models have been deemed 'unbalanceable' due to their size and shape.
While I think the current coalition designs are better, they are not functional. These models are a more conventional shape and will be easier to balance.
These ships belong to the Coalition, I think it should be up to them whether they think their ships need a replacement, no? If you're not in the coalition, don't bother saying "they don't need a replace, I think the current ones are better etc." because it's really not up to you. After all, they're the ones who fly them.