Keeping a small squad, then switching to new soldiers (from missions and/or hired) after buying Iron Will is a good way to mitigate this, if higher Will is desired.
Soldiers receiving promotions after the upgrade is purchased will receive its benefits, but not the ones before.
While expensive, New Guy ensures even the lowest soldiers will have a Class unlocked which in turn grants access to the full range of high end weapons during missions this is particularly helpful in counteracting the frequent losses and injuries among veteran soldiers on higher difficulties.
New Guy retroactively applies to all to current XCOM Rookies.