First of all hardbat was great when it lasted till 70 years ago in 1952 when sponge arrived.
Secondly sponge tabletennis rackets are still not available in some countries
That said, it is somewhat psychotic to try turn the clock back just because some hardbat / sandpaper players who cannot handle the complex spins of sponge tabletennis. Turning tabletennis into pickleball still won't bring millions of spectators to the sport. The sport would still remain a participant sport for the most part & not a spectator sport.
That said, What are the biggest advantage of sponge tabletennis over hardbat / sandpaper ?
Hardbat / sandpaper is a one size fits all concept. This does not recognize the simple fact that the forehand & backhand of 90% or even more of humans is very different. Most clueless amateurs get confused however watching majority of pros having equally successful & same type (looping) forehand such as the likes of Ma Long etc. But this is not reality. Also for most humans forehand is the stronger side by nature in any racket sport but is all the more significant in tabletennis because of very diverse playing styles, again compared to other sports.
Hardbat & sandpaper do not accommodate for these differences. But sponge era rackets allow far better for the expression of individuality & enhance the specific & different skillsets on forehand & backhand far far better than hardbat / sandpaper can.
Are there problems in sponge era table tennis ? Of course there are. But regressing to hardbat is a psychotic solution because the most distinguishing element compared to other racket sports is its compelx, diverse & extreme spins. And most younger players would prefer & would prefer sponge tabletennis if they enjoy creating & solving these diverse spins.
But hardbat players & roboTnazis of course will want to use ONLY long pips & anti rubbers as an an excuse. Chilren of course should start with spinny inveretd rubbers to learn the fundamentals. Going to hardbat & or even worse switching between hardabt & spinny inverted is a very idea. Sure the most talented but few juniors (likely future stars) can adapt to this switch back & forth but most junors cannot.