Lots of good points made already. As others have said it's really a 'horses for courses' type question. I would add two things: When I used to do monitors for a living, prior to getting kicked upstairs, I found IEM's were incredibly helpful if the turn had a weak singer (no doubt the singer found it incredibly helpful too, but I'm being selfish😂). Also, in terms of monitor systems, I think from memory all of the acts which I did used IEM's as an add on to their 'conventional' monitor systems. Most specs I see nowadays seem to ask for both. I don't recall ever seeing a spec for a silent stage, but I guess this is what Metallica do, so who knows?
Speaking as a player, and one who preffers wedges to IEM's, the big revelation for me came in the form of our drummer starting to use a triggered kit. All of a sudden there was no need to keep up with an acoustic drum kit. The on stage volume at shows and rehearsals is very much more civilised, and the complete lack of the aural 'slosh' (for want of a better word) you get onstage from live cymbals and hi hats makes a huge difference for the better in terms of listening fatigue. Our monitor system now comprises 4 Eon 110 speakers with an added 12" sub for the drummer. Works a treat. Having said all that we still have to carry all of this. IEM's win hands down for portability.