How it was supposed to work:
You reach 100% of your power through normal gearing. Content is tuned for 100% of your power. Legendaries and Titanforging were boosts that would push you to 120% of your power and let you demolish everything including mythic raids. That's how things turned out in the Emerald Nightmare.
How Blizzard implemented it post Emerald Nightmare:
You reach 80% of your power through normal gearing. Content is tuned for 100% of your power. Legendaries and Titanforging, despite being RNG in nature, are mandatory to barely struggle through mythic raids. That's how things turned out post the Emerald Nightmare.
There is a reason more than 2000 guilds killed Xavius mythic while Kil'jaeden mythic sat at around 700 guilds.
How it works now:
Legendaries are not really "legendary". Rather than being nice additions on top of your power, they are completely mandatory. Acquiring a legendary doesn't feel like you are powering up your character past the limit. It feels like you are reaching the level of power that you were supposed to be at baseline.
The problem was never the RNG aspect of Legendaries and Titanforging. The problem was that Blizzard thinks that endgame should be e-sports reserved only for the top 0.1% of players. If mythic raids were tuned to baseline gear with no legendaries, people would be able to power through them after acquiring good Titanforging procs and Legendaries and we'd have thousands of guilds achieving CE every time. But according to Blizzard, that would be a problem because some people find busting their heads in to clear a bunch of pixels "fun".