Just finished skimming this lengthy thread and I have a few comments. First of all, I have never dove the Doria nor have I climbed Everest, but I do wreck dive and I do mountaineer [but not at levels anywhere near that required to dive the Doria or climb Everest] .
Diving and climbing are fascinating to compare as both involve: physiological limitations imposed upon humans by their environment, extreme/severe environmental conditions, proficiency with technical equipment and use of sound judgement. Both disciplines dish-out severe penalties for exercising poor judgement or pushing your limits too far too fast.
Mount Everest is the tallest point on Earth, that has cachet. Everyone knows about Everest. The standard route up Everest is by no means hard, technical mountaineering, but it's Everest ... it's the highest place on Earth and that immediately gives it more 'cachet' than it's far more difficult stratospheric neighbour: K2. The world's most accomplished alpinists balk at Everest's standard route and refer to it as a 'yellow brick road'. Technically it's a joke compared to other mountains but, the altitude and extreme wind/weather is what makes Everest's standard route dicey. Retreat from high-up is difficult too. The vast majority of the deaths on Everest occur on the standard route and usually it's related to poor judgement, lack of experience, and bad luck ... or a combination of these. It's been said that being able to cough-up the $65K to climb Everest is the only prerequisite. Despite being technically easy, Everest's standard route still enters the 'Death Zone' where there's 1/3 less O2 than at sea level. You can imagine the effect this has on a less-than-adequately experienced climber even with supplementary O2. Lots of people climb Everest not for the challenge [because there are lots of other more challenging routes/mountains out there], but because they can say 'I climbed Everest' and people will immediately recognize that. If someone said 'I climbed the 'Rupal Face' on Nanga Parbat', which is an exponentially more impressive feat, this would mean something to mountaineers, but the average person at a cocktail party would have no idea what they were talking about.
The Doria has always been referred to as the "Everest" of diving. That is, it has the 'cachet' of Everest when it comes to diving. It's by no means anywhere near the deepest part of the ocean ... if we could physically dive the Mariana Trench in a drysuit and doubles - that would be the 'Everest' of the diving world. Let's face it, there's far more dangerous diving to be had: much deeper wrecks, long+deep cave penetrations etc. Take the Britannic for instance: it's of the same lineage as the Titanic [worlds most famous shipwreck and most inaccessible], it lies in 400 fsw [almost twice the Doria], and very few have the skills/abilities/resume to dive her.
The allure of Everest will never waver - it's the highest point on Earth. But why is the Doria known as the Everest of diving. Sure it's hard: it's deep, it's cold, there's current, it's 50 nm from land. Perhaps it's because people have been diving on it since 1956 [the day after it sank, with the sweet equipment available in 1956 and no fancy gas mixtures] and it's now had time to earn its reputation. People just haven't been diving wrecks like the Britannic for that long. People have been diving the Doria for 50 years and she's claimed many lives, this is why she's become the 'Everest' of diving - the death-toll adds to the allure and divers want to test their mettle. A lot of people have dove the Doria despite it being considered a challenging wreck dive [same goes for Everest], this means that it's an attainable goal, if not a realistic goal once the skills and extensive experience has been acquired ... this adds to the allure. For the truly talented/experienced diving elite, the Doria is a right-of-passage, the same holds true of the standard route on Everest for elite mountaineers.
In the end, referring to something as 'the Everest of' doesn't really mean it's the hardest of the discipline [whatever that discipline may be: diving, sailing, climbing], but it's the most prized or recognized for whatever reason(s).