No that’s not true.
Rank 1 is given to the top 0.1% with at least 150 wins. That makes it possible to calculate how many active Arena players there have been in a given Season by looking at the amount of Rank 1 titles handed out.
MoP’s most popular Season was Season 15 with 375k eligible characters.
WoD’s most popular Season was Season 16 with 320k eligible characters.
BfA’s currently completed Season 1 had a total of 405k eligible characters.
So BfA is currently the most popular Arena has ever been.
And you all know that, because there’s been countless threads on this forum during Season 1 where people have commented on how everyone and their mom are getting Gladiator and how Rank 1 titles are just being handed out left and right.
And that’s because there are a lot of people participating. That means lots of spots for Gladiators and Rank 1s.
And if you factor in how alt-unfriendly BfA currently is compared to MoP or WoD, then you really get a sense of just how popular Arena in BfA is.
Disagree, but I’ll accept the premise and reiterate what I said earlier:
Class design is objectively better than ever before, because there are more classes to play, more viable specs to choose from, and more comps to explore. And the addition of Azerite Traits and Honor talents introduce more character customization as well.
The sum of the above is objectively better than in years past where you had less of all that.
People who are happy to play Demon Hunter today are inevitably better off than in WotLK, because they couldn’t play Demon Hunter back then!
People who are happy to play Fire Mage today are inevitably better off than in Legion, because Fire Mage wasn’t viable to play back then!
Good class design is something that is as popular amongst as many players as possible. It’s not about having an Arms Warrior design that perfectly caters to Saiyoko’s refined tastes. He’s just 1 guy. If there’s another design that appeals to 2 people, then that design is objectively superior.
The needs of the many outweigh the needs of the few, or the one. 
My argument is that the volume of viable specs and comps in Arena (and PvP as a whole) has increased over the years since TBC when Arenas were introduced.
I find that very difficult to be a disputable argument, since Blizzard have introduced 3 brand new classes to the game since then (not to mention all the new races with their unique racials), meaning that PvP is ultimately much richer today than ever before.