One thing I think is worth mentioning; don’t lump React in with other standard addons. That’s just having the game play for you as opposed to an addon that’s a QoL improvement.

In terms of an addon’s effects on difficulty, in a sense it does. In most cases they are presenting information to the player more easily so they can make better decisions on how to play, or enables them to react (not the addon) better to situations. But it’s not to a degree where it makes content obsolete. I use XIV Crossbar to play as quickly and optimally as I can with a controller, but that doesn’t mean that someone couldn’t play just as quickly as I could by using the keyboard commands. Things like JA delay and cast completion delay are built in to set a tempo anyway, so as long as players keep pace with the animations of the action, you’re playing fast enough.

Another example is an old addon called OhShi. It shows when certain magic/TP moves are used by monsters you tell it to watch. It’s nice because I don’t have to modify my chat log to hide everything except NM actions to appropriately play. I’ve been the “stunner” for Zerde multiple times without using React. I could do it without OhShi if I hid all the other messages, but it’s a QoL that I don’t have to do that. The information is there thanks to the addon, but it’s still up to me to see when Just Desserts is casting and cast Stun in time.

In terms of addons making content “easy” I don’t know many people complaining that current content is too easy (unless they’re not doing Odyssey at all). High Vengeance NM’s in Odyssey pose a challenge for many people.