Let me break down your post and address all of your points. Not trying to be nasty here, just clarifying some things about WOW's addons.
Reason being is that I could have sworn I have read some WoW developer saying that their allowing of add-ons make balancing content extremely hard (unfortunately I can't remember WHERE I've seen this).
What you've probably heard is that Blizzard is purposely designing boss fights knowing that the majority people are going to be using addons like threat meter, Deadly Boss Mods (timer/boss ability info), and some sort of custom UI that caters to their play style. This doesn't really mean anything, because Blizzard themselves have integrated all of these addons into the default UI (much like how FFXIV is doing it). WOW now has a built-in threat meter; you get shout, tell, emote, or visual warnings in the middle of boss fights about upcoming events; and WOW's default UI has seen massive improvements.
In actuality, Blizzard basically said that they will be moving away from bosses that have random raid-wiping abilities and move towards a model that tests twitch reflex, pattern memorization, raid makeup, and gear. It doesn't make content harder to balance at all.
A boss that's a challenge for people without add ons is stupidly easy for people with the right add-ons.
I didn't think this was the case. I was a tank in WOW for 4 years and I played on and off with addons; I never noticed a difference. Mostly because like I said above, there is more than just knowing what a boss does and when it does it, if you want to beat the boss. If I backed myself in a stupid position, if my gear sucks, if I'm tired and my reflexes are slow, or if my raid doesn't have the right classes or interrupts, it doesn't matter if I know whether a deadly ability is coming up or not. I'm going to die, and the raid is going to die.
I stopped using threat meters in WOW, and only rarely used DBM, and I was still able to be in one of the better raid guilds. After I learned every fight, DBM did nothing for me.
I'm not sure how it is in WoW, but I don't want to practically be forced to use third party add ons so I can spam one button to automatically find a guy in a 40 (?) man raid who has a curse debuff from the boss so I cast my decurse spell on him automatically (That's what I've seen my friend did).
Two things here.
One, you are only forced to use an addon because of social pressure. It was used as a safety net of sorts early on in WOW's history to make sure that players new to a raid were paying attention, not ******** simple things up, and learning the fights. You can get by through all of WOW's boss fights today without addons, provided you know the fights. See above.
Two, the addon which you're talking about that let someone do a "one click heal" is functionality already available with WOW's default UI and macros. WOW's default UI has raid frames, which gives you a compact list of all 10-40 players in your raid, and macros allow casting on "mouse hover" or "on click". And it's actually how HealBot (is that what it is now?) and Decursive work; they create their own macro entry in your macro table, and reskin the default WOW raid frames to let you do "mouse hover" and "on click" actions.
My point is, my impression of add ons is that they give the players the freedom to customise their UI, but at the same time I've seen add ons that make playing the game almost pointless (like my example), or the add ons are almost required to be the most 'efficient' in fighting that particular content, and the fight may be almost impossible without the add ons.
Any WoW players can attest to that? (I think add ons are allowed on very few MMOs, WoW being the most popular of them with tons of add ons, and that's why I go specifically with WoW.)
Bottom line, WOW's addons don't let you do anything WOW's default UI doesn't. They simply re-organize and re-skin information that's already available to you. Blizzard has been pretty quick about stamping out any addon that automates gameplay in any way. I wouldn't worry about it.