Not quite ELI7, but....
The growth from 50 to 80 isn't linear. The curve gets steeper the higher you go. That is, the amount of growth between 50 and 60 (10 levels) is not the same as the growth from 60 to 70 (10 levels), and neither is 1/3 the growth between 50 and 80 (30 levels). The difference is much greater.
Here are some hypothetical graphs:
The top row is an example of the current power growth curve. The heights of the pink bars show the power at a lower level, 50 or 60, and the heights of the blue bars show the power at a higher level, 60 or 80. You can see that the difference between overpowering Lv50 content at Lv80 is several times more than overpowering Lv50 content at Lv60.
The bottom row shows a hypothetical stat squish. In this particular example, the post-squish Lv80 is about 1/3 the power of pre-squish Lv80. Notice that pre-squish, Lv60 was about 2.5x as powerful as Lv50 but post-squish Lv60 is only about 1.5x as powerful as Lv50. Likewise, pre-squish Lv80 is about 16x stronger than Lv50 and about 6x as strong as Lv60, but post-squish Lv80 is only about 6x as strong as Lv50 and 3x as powerful as Lv60.
The new power curve would mean that a Lv80 doing Lv80 content is still relatively the same pre- and post-squish, and a Lv50 player doing Lv50 content is still relatively the same pre- and post-squish. But compared to unsyncking content pre-squish, the post-squish Lv60 player doing Lv50 content is about 4/5 as powerful as they were before, while the post-squish Lv80 player doing Lv50 content is about 1/3 as powerful as they used to be.
Of course we have no idea how steep the new curve will be in reality. This is a fairly exaggerated example to make the concept easier to visualize, but hopefully this explains why there would be a difference at all just for unsyncked players. This could very well mean that things some players are used to soloing might minimally need to be duoed, or maybe a duty you can currently duo might minimally need 1 each of tank, healer, and DD after the squish.