Why can’t Blizzard implement a monthly update system? with every 4 months a larger content update?
Monthly trickle changes to the over all story and then larger mechanics and systems changes.
This would be far better than their really obvious gating systems they have now to keep us “subscribed”.
If it were focussed on small continuing story elements and maybe the occasionally fun cosmetic things it would be more engaging to the community.
Marketing has their head so far up their rears that they cannot comprehend how a game with this strong of a following can still be successful without the need of arbitrary lockouts and delays.
Because this game has no many problems with balance and bugs that it would be impossible, people are discovering new things that need to be fixed daily so a system like the one you suggest wouldn’t work for a game of this scale. Also they want you to spend money on expansions.
If your “nitpicky” about it… they kind of already do? 
I feel that’s why there are so many “holiday” events going on during each month. Is to try to keep players occupied with different activities like Brewfest (Example), Timewalking or the Darkmoon Faire.
They also kind of do it in the way they timegate most of their patch content. As not wanting players to finish it all within the week they timegate the quests to be spread out through multiple weeks.
In this regard you could call Blizzard lazy, which is partly true. But at the same time I think they’re not wrong in thinking if players had the option to do everything within a week or two, a lot of players would and that would cost Blizzard $$$.
For a multi-million / possibly billion dollar company, they do have the tendency of a poor indie studio sometimes. Why? I couldn’t tell you. 
your assuming here that timegating is to compensate for a lack of updates. in actuallity its to make the endgame more accesible and to make you more likely to keep a subscription long term.
A patch is not something you can just make and deploy. There is a lot of technical testing that is required.
Having patches, even smalls ones, any more frequent then they are would be dramatically more work.
And for what benefit?
If you need new content every 4 weeks, this may not be the game for you.
No I don’t think having ‘set time to do an update’ as a concept works very well. As long as they create new and engaging content and deliver them in what would generally be considered reasonable is all they should be aiming to do.
I haven’t been involved on a large scale MMO but I have been a part of the release a smaller Asian mmo; Dream journey online (2015-2018). We did have the same mentality - a monthly theme whether it was a tier spending event, new free ingame content, etc. It’s nice for the playerbase but in my opinion it led us to rush more content rather than focus on improving the game/fixing bugs/improving design concepts. With that said it’s very likely this company has significantly more resources so they would have a greater ability to assist with your idea.