There are two issues in deciding the which language is the “new Yiddish.”
Yiddish eventually became a uniquely Jewish. Also, it was the most widespread primary way jews of all stripes understood each other in the latter half of the second millennia of the common era.
The question: if a language has only one of those two qualifications, can it be the “new Yiddish”?
Next question:
If you take a random grouping of 1,000 known Jews (defined by orthodox tradition) and say a sentence in Hebrew, Yiddish, and English, which language will be understood by the most people?
I’d guess English >> Hebrew >> Yiddish but I’d need to do more research on the demographics of known Jews.