I strongly disagree with your post. First of all, GR XP was not changed, but it behaved that way for a very long time (if not all the time) long before the XP farm methods got popular, there was just no one doing it for two reasons: 1) Multiplicative XP bonus was widely unknown (even among top players) and 2) Running GRs was pretty crappy because of RNG, which got significantly improved in 2.1.2.

Another point is that the choice between "good drops" (T6) and "good xp" (FoM) was one done by the community, not by Blizzard. While it's true that it's necessary to have an offset if you choose one over the other the way it worked before was not optimal, because farming goats all day was an incredible dull, repetitive, and unrewarding activity that people had to do if they wanted to keep up on the paragon farm. Even in very efficient GR groups you usually can't achieve the same number of legs per hour as you can in very efficient T6 groups.

Also, your statement about class diversity is not true. If you fail to get accepted to a proper group, create your own one, but don't blame the community's stupidty for a problem that does not exist. Every class save has it's spot in a XP farm group, even a freaking wizard. All you need is at least 1 DH to kill the RG. In fact, Barb is a really good addition to such a group and usually a must-have for the most efficient farming.

There are however still three main issues that you failed to address:

1) One of the supporters (mostly monks) can go full leech and equip all the XP items. This is a problem because one member of the party gets boosted like crazy while also running the most efficient setup possible (which is full support). This will lead to those classes (esp monks) being at the top of paragon and eventually character strength and requires that people racing for paragon play this class.

2) One of the party has to provide the keys. While this is a good way to offset the huge benefits of running GRs, it is also one that is only relevant to one player of a four player party. The rest just reuses their keys. This means that there always has to be "this guy" who has to take one for the group.

3) Solo players get screwed in this system. While they got screwed a lot in the past already, this has gone to a whole new level now.

To sum it up: I think the changes to GR and the "meta" are very well done. I really enjoy the new GR farming methods and I think the XP bonus should stay. Blizz doesn't need to increase the Torment levels because that is exactly what greater rifts do, and you can even choose precisely how difficult you want the game to be. There still is work to be done but it's on a good way, and T6 still has its place.