Is SM taking so much time to debut their 4th gen BG because of NCTzens ?

Nah, it's not NCTzens, it's SM reshuffling its plans. I think the pandemic threw everyone's timing a curve ball, so they've been scrambling and rearranging the schedule since. In the original plans, NCT 127 would have dropped their album and been touring for 2020, with other groups also promoting. Everything got reshuffled and remade and it's been a moving target ever since, whether it was NCT 2020, albums for all the sub-units or even plans for other artists within SM. The massive success of Resonance, as well as Sticker/Favorite and Hot Sauce/Hello Future have further reshuffled and shifted plans as they seek to capitalize it, as well as the WayV scandal, which caused a tour and an album to be delayed.

There's no need for them to rush when there are multiple viable groups, including whatever new NCT unit will support Shotaro and Sungchan.

I also don't think they're too worried about a surplus of talent driving decisions - members like Jungwoo, Kun and Johnny were trainees for additional years. They debut them when they're ready..

Not happening until sm debuts sungtaro unit and nct Hollywood

well they will debut a new boygroup soon now i think

with nct127 enlisting (starting with taeil this year i believe) and contracts up next year (7 yrs since nct debuted in 2016) they can't afford to put off plans for longer