I have always been an altoholic, to experience all the aesthetics of the starting zones and have multiple sides to the story/build-up. On the Horde side naturally :P I was doing some raiding with the guild(s) all the way from vanilla, but in WotLK I started raiding seriously. And I also started running rather successful PUGs weekly on a few alts, thanks to the handy calendar feature. I think I had about 6 raid geared alts back then.
With the introduction of LFR in Cata the gearing of alts got even easier, no need to try to PUG stuff anymore (also PUGing on my server had become such a chore due to population dropping massively), and the number of geared characters got to 10. However I can't say they were seriously raid geared, but geared still.
MoP stayed pretty much on the Cata level, but added 1 more toon to be easily geared with the monk.
In WoD things started to feel painful. The leveling alone was annoying to me on multiple characters. Only saving grace was leveling only when rested and skipping a lot of the stuff you had already done for a gazillion times. Added to that, it seemed like leveling in PVP had become extremely slow. Queues didn't pop fast enough anymore, and even when they did, even winning didn't net enough XP to warrant the time spent in the BG. And that holds true for Legion too.
At least in Legion one can choose the order of the zones. Feels like the story part leading to the Pillars of Creation for each zone still is a thing that must be done, especially before they let you have world quests immediately at level 110. And boy oh boy, there sure is a lot to do at 110. And I'm not even being sarcastic here. It's just that it feels so much like busywork. You do world quests that offer items with baseline item level that is 20, even 30 below what you would need for an upgrade, and pray for the Titanforged proc to happen, and then be high enough to actually matter.
It's tedious. Not rewarding. Even legendaries from caches/dungeons/LFR are just a momentary happiness. Pure RNG doesn't feel even a bit rewarding.
One can sink in hours upon hours on several alts to clear the map of each and all world quests only to receive 1 or two minor upgrades across all toons.
And then one just let's the subscription to lapse. At least I did. It no longer felt fun. In fact, it hadn't felt fun for a while, it just took too long for it to really register. It's not about the amount of content, for me that is absolutely great in Legion. It's all the busywork. Even the professions, crafting feels useless. And all the busywork there too... and RNG.
I know, I know, it's all anecdotal and blablabla, but I'm not speaking for others anyway, just sharing my perspective on all this. More to the point, people on my Battle.net list used to be all playing WoW, after all, that's how I met the people. Now, if they log on at all, they play Diablo 3 or Hearthstone, not WoW.
The world seems full of people though, at least in the WQ areas. But they are not people from my server. And neither are the people in the cities. I would rather see realm mergers than all this artificial CRZ nonsense.
At least for me the activity in the open world feels just like it's only smoke and mirrors.