...is because the last big Boygroup to debut (Enhypen) will soon 3 years old
It's a big age gap. The current big BGs are all very well stabilished.
The next BGs will take a couple of years (at least 3 or 4) to reach TxT/SKz status, by that time both groups will likely engaging on their enlistment duties
Imo, BGs are the ones who really mark the start of generations simply because the biggest BGs debut early (to acumulate more fans and drive sales). The top 3 BGs on 3rd gen (EBS) debuted on average 2 years before the top 3 GGs (TBR). In 2nd gen was similar with TVXQ and BB debuting 3 years early than SNSD/2NE1
So we are seeing the starting of 5th gen now in 2023 and 2024, while 4th gen BGs still strong and 4th gen GGs dominate the charts. Except by ZB1 who has a huge fandom pre debut I don't think any of the newly debuted BGs will have huge sales, they will grown with time and maybe even take a couple of years to sell 1 million
In a couple of years (3 I think) we will see a shift, when the biggest 5th gen GG debuts and the biggest 4th gen BGs start to sell millions of copies
Just my 2 cents