Demo is balanced in the current 6s meta because 6s is the community's attempt at contriving a balanced version of tf2 without straying too much from the original game (pubs). This is why valve will never (and shouldn't ever) balance the game around comp: the "true" form of the game is what's played in pubs, and we should adjust the format for any major changes.
Hence demo can be balanced in 6s like the queen is in chess, but in pubs where you can have 8 queens it's not so fair. A 9 class system is inherently a nightmare to balance because you have to find 9 different important niches, one for each class to fill, and make all classes relevant.
Demo makes other classes such as soldier irrelevant (assuming a medic is on the team/gunboats notwithstanding) because he can fill the same mobile damage role but overall more effectively. The "glass cannon" argument doesn't really work because he has a significant amounof health and enough pipes to kill/hold off an assailant until his team kills them for overextending. As such demo is currently just a damage machine, which we have taken into account with class limits.
In the end a demo nerf isn't super pressing for 6s because his strength isn't unbalanced (both teams have one) and also have lots of offensive classes (no defensive bias). In HL it's more important because while both teams still only have one demo, he can be more easily protected because of the addition of pyro and heavy. In pubs, where valve should balance around, demo is ridiculous because it makes other classes redundant and if a pub team were to work together it would be almost impossible to push a last against 2 meds and a stack of demos (huge defensive bias).