While I can agree with the plea "Dont make current content easier", I cant agree so much with "Old content need to stay the same difficulty that it had when it was released".

Meaning: As long as we get new, difficult content - and we do, even though Byakko for example is a rather easy EX-primal, Shinryu wasnt - I'm fine with the old stuff being made easier.

Horses had their time to be special. Birds had their time to be special. Now dogs are special - if you want to feel special, get your Shinryu dog and dont point towards your 3-year-old Garuda pony!

If you want something bad enough - either because you really like it or because it makes you feel, well, cool and special - you'll put in the time. If not, you'll get it later.
I'm fine with that - and I'm sorry, but you sound like you dont or cant bother with the new content in the game and therefor feel the need to preserve the old content in a way that still makes you feel like you'ce accomplished something. We had this kind of discussion before. The problem here is: Its outdated, old content. People do it unsyched - or not at all.
We got new content, new challenges - if you want a challenge: Move on!
They're still in the game!
You have no dogs, not a complete set of birds, non of the mounts from the new raids...

Sigma Savage - and more so Ultimate Coil of Bahamut! - is far from being "beaten by everyone". I dont know where you're getting this idea. Honestly, you sound as if you used to raid and felt pretty special and hardcore about it and now that you dont anymore, you try to wave around old achievements and want them to stay extremly valuable because you cant get the new stuff.

Increased mount-droprates are keeping the old EX-primals alive.
More MGP are "needed" because there are more rewards.

And I still have the same feeling I had when killing T13 two weeks before Heavensward that I have today when clearing current Savage-content.
But obviously, to get this feeling, you'd need to do that content...