This is exactly how we got here in the first place.
I'll be blunt here. There is absolutely no way they can do this.
If someone plays a tank, they're probably looking to be a leader, a protector, there's no way to entice them to play a squishy job that supports people. Same for DPS, they probably want big numbers and low responsibility, there's no way to make them pick up a healer.
If they really want to boost healer numbers back up, they should start catering to people who want to play healer, not rework the entire role to try and lure people who aren't interested in healing.