I'm not sure I buy this. There are tons of competitive decks that only use a few high end cards. Legend caliber Zoo and Hunter decks can be crafted from scratch for under 1500 dust. There are Priest and Mage decks that have ~2 legendaries in them. Rogue gets by with just Thalnos and Lotheb now. Shaman and Druid aren't legendary heavy, although their epics are required. It's pretty much just Warrior, Pally, and Handlock decks that use 10k+ dust. I mean I guess you could make the argument that a lot of the decks use different ones so it's hard to have enough to play all the decks. But even then there are a few that translate really well into several decks.
I've been pretty lucky with packs lately, but the legendary drop rate still looks like ~5% for me. Although this weekend I opened 15 packs and got 4 legendaries, 3 epics, and 5 golds which I wasn't complaining about haha.
I second this opinion. I really don't think that the current meta really requires a lot of Legendaries to be competitive. Most of the legendaries you do you see are common to a vast majority of decks. I'd break it down like this:
Frequently seen in competitive decks:
Loatheb
Sylvanas Windrunner
Ragnaros the Firelord
The above 3 are staples of a vast majority of competitive decks. Loatheb and Sylvannas in particular, largely due to their extremely unique effects, and the difficulty required to play around them.
Commonly seen in competitive decks:
Alexstrasza
The Black Knight
Cairne Bloodhoof
Ysera
Outside of Control Warrior, you will probably never see all 7 of the above legendaries in any single deck.
Occasionally seen in competitive decks:
Bloodmage Thalnos
Harrison Jones
Leeroy Jenkins (there has been a massive falloff in his usage since the nerf, but he's still out there in a few hyper aggressive decks)
Kel'Thuzad
This set of legendaries appear in a fair number of decks, but due to the current meta and how difficult it is to get the most out of them, they are pretty rare.
See only in very niche decks
Illidan Stormrage (starting to see some usage in demon based Warlock decks since the introduction of Voidcaller)
King Mukla (seen only in hyper-aggressive Hunter/Rogue decks, and even then its pretty rare)
Malygos (starting to see some usage as a new version of the 'Miracle Rogue' deck, after the nerf to Leeroy)
Baron Rivendare (rarely seen, but some people are trying to experiment with him in various Deathrattle decks - no real success yet)
Feugen/Stalagg (similar to Rivendare, people are trying to figure out how to get use out of this combo, but as of yet there's been no real success)
This set of legendaries has seen essentially no real success in competitive play. You'll see a decent amount of them on the ladder as people experiment with them, but only in the lower ranks, because they just aren't very good, or people haven't figured out how to use them properly. They certainly aren't requirements for putting together a 'good' deck.
Class Legendaries:
Al'Akir the Windlord - This is the staple card of most high level Shaman decks, it will be in almost every one, but due to Reincarnate and all of the new deathrattle cards, there are plenty of new Shaman decks coming out that don't require him
Archmage Antonidas - Starting to see some use in Freeze Mage and Secret Mage decks, but overall its still near the bottom of the list in terms of 'must have' legendaries)
Cenarius - outside of late game heavy ramp druid, he is absolutely unnecessary in any deck
Edwin VanCleef - You'll see him in both of the popular Rogue decks (Backspace, Miracle), but he is not the cornerstone must-have card of either
Grommash Hellscream - A long time staple in the Control Warrior deck, but that deck already requires 6-7 other legendaries anyways, so you shouldn't even be looking at Grommash until you've amassed quite a lot of legendaries or dust
King Krush - Meh. Never has, and barring some balance changes, never will be required for any deck.
Lord Jaraxxus - After the Leeroy nerf, Jaraxxus has been seeing a lot more play in Handlock decks, as well as some demon-based decks (after the introduction of Voidcaller). Handlock fortunately is pretty light on Legendary cards though anyway, and Jaraxxus is certainly not a make-or-break card for the deck.
Prophet Velen - Widely considered one of the weakest class-based legendaries, it has never really seen wide spread use out outside of very gimmicky decks.
Tirion Fording - An absolute staple of the Control Paladin, but not necessary for aggro or mid-range Paladin decks.
What really matters in all of this though are the first two sets:
Loatheb
Sylvanas Windrunner
Ragnaros the Firelord
Alexstrasza
The Black Knight
Cairne Bloodhoof
Ysera
With just a few of these 7 legendaries, you can make a multitude of high level competitive decks. And Loatheb is even free (assuming you beat the Naxx wings)! The only competitive deck that actually requires all of these is Control Warrior. With just 2-3 of these you can make almost any high level deck out there (occasionally you'll need your class legendary as well). And much of the time when you are trying to make a deck where you are missing just one of these legendaries, there is usually a similar non-legendary out there that you can replace it with without losing much power.