There was once upon a time esp during the Donic Bluefire M1 era, its 47.5 degree hardness ( ESN scale ) is considered hard sponge. These days ESN company are upping the ante by going towards 55, 56 and 57 degrees. It is getting harder and harder.
ESN Hard back then (47.5) to me was way too soft for FH.
The direction by pros, and amateurs since 40+ is to go harder sponge.
in the 38mm era, soft soft soft was in demand.
40mm, T05 and equivalent was okay
40+, Butterfly still only had a T05, but pros has firmer sponge version for years.
Then T05 Hard came out to fit the requirements all the way down to consumers.
ESN also started to go extra hard and entering the 5x space.
TT has changed a lot due to ball change. It is only normal rubber makers need to adapt, or risk loosing market share.
DHS (the biggest ball maker in the world) funny enough, is one of the beneficiary of 40+ ball change with its sales of H3 going up the roof