which top 3rd gen group that will definitely NOT SURVIVE if they debut on 2nd gen ?

Most of them

Aren't BTS, EXO, BLACKPINK, and TWICE the only top groups in 3rd Gen? Since there are still a lot of K-Pop fans who don't consider SEVENTEEN as a top group because they are far behind from those 4 top groups in terms of digital performance, even though they're the 3rd biggest boy group in their generation. Also, I think RED VELVET can be considered as a top group, since they're in the overall Top 5 in 3rd Gen.

As for OP, I think those 4 top groups will still do great, considering that their digital performance are outstanding, and 2nd Gen is all about digitals compared to 3rd Gen and 4th Gen groups which is more on physicals.