That’s right and don’t forget that Sam and dean really have nothing left to do in their world. Their both dead and Buffy is not, the only way I could see them making an appearance is if the GOD glory came back and revived sam and dean and sent them to the Buffy verse with a lie

Yes but don’t forget as of now all characters in each show are around the same age gap, while Buffy was doing her thing sam and dean were on other respective shows so technically Buffy is only like 7 to 8 years older than them when Buffy started she was 17 dean was like 11or 12 at the time so the age is not that drastic it’s just a blending of their two different realities that has to happen