Personally, I think there would be would be pegasi that would be able to pull off a sonic rainboom - if only because she did.
There's nothing that spurs challenge in something previously thought impossible than finally proving it possible. The rainboom was originally thought a myth, or at best a theoretical exercise, but once Rainbow Dash pulled it off, and then proved it wasn't a one-time fluke, you can bet that there would be pegasi dedicating their lives to training to do the same, then outdo it.
It might be an interesting situation to explore - how would RD react? Would she feel threatened that her monopoly is in jeopardy? Would she be proud that she inspired what was basically a new era in flight? I'd be curious to see.