Back in the day before high tech lines, wire halyards were used to reduce stretch. Modern lines are as strong or stronger than the wire halyards with little or no stretch. They are also lighter, easier to handle, not prone to fishhooks, easier to splice, and cause less wear and tear on expensive parts like winches.
There are different opinions on changing halyard sheaves. Sheaves for wire halyards have a primary groove to accommodate the line portion of the halyard and a narrower groove to accommodate the wire. If the sheave is in good shape, smooth no cuts or rough spots that would chafe the line, and turn freely, then is it probably not necessary to change the sheave. Older boats often used an aluminum sheave that rode on an aluminum sleeve around a SS bolt. Corrosion occurs which inhibits the sheave from turning freely. Dragging a loaded rope halyard over a frozen or partially frozen sheave can be damaging to the line, so the sheave needs to be changed. Axles and sheaves can also develop flat spots which restrict the sheaves ability to run freely.
If the mast is down, changing a sheave is usually no big deal. Installing a new ball bearing sheave make raising and lowering the sails much easier. Garhauer Marine makes very affordable sheaves and other rigging products.