most people saying that, from what i have seen are only fans that joined in this generation or shortly before it, so they are jumping the gun a little bit not realising that in all the generations there have been outliers... for example a lot of people think its weird to group stray kids and bts together so they make the seperation around that time, but you're always going to have a few groups who debuted at 'the wrong time' and are sort of imbetween
the 3rd gen groups are in their prime still, though some are starting to reach the 'expiration date' so we can tell the third gen will be reaching its end relatively soon, but its definitely not quite over yet. Twice and bts for example, are third gen groups, how can you cut the generation on Twice a year after their debut? (yes some groups are outliers to the time frame but twice is unanimously recognised as the gg of the 3rd gen)
plus 19992 is when seotaiji debuted 2003 is when dbsk debuted 2012 is when exo debuted
some of the bigger groups life spans definitely influence the dates of generations.
infact my 2019 date is tentative because it sort of depends on how long it takes the newer groups to solidify their presence, and depends on how much longer the third gens continue going strong with only themselves as competition.