Addons can certainly cause problems. They run in a fairly slow language and there is a lot of checksums and verification involved, so if you ask them to do something very difficult they will actually tank your FPS. I should know - I’ve made addons that tank my FPS myself.
But you’re right about one very important thing: the reason we install all these addons is because the game isn’t payable without them, and worse still the reason they aren’t is that there’s too much happening, which ironically causes even addons to get clogged when trying to resolve everything to give you enough information to make the game playable.
Blizzard simply must clean up the gameplay. It is the only way.
I said it a long time ago, and it’s even more true now than it was then.