Most of the legendaries are pretty bad. At most a quarter of them are worth using in the meta and most of the competitive decks use between four and eight of them. It's not like you can just say "this deck needs five legendaries" and then stick any five in there. The odds of opening even a hundred packs and getting the cards needed for a competition level deck are pretty much nil, with the exception of mech mage as it only really requires two expensive cards (and people complain that everyone plays mech mage; wonder why that is).
I opened 60 packs and got Gruul and Baron Geddon, that gets me nowhere. By my estimation, the investment needed to be able to make, say, three competitive decks is in the neighbourhood of three hundred euros. From then on, the cost of further decks is smaller as some of the legendaries are re-used and you'd have most of the rares and epics from opening 200+ packs, but that's also an absolutely insane cost of entry to a game where the "journey" has essentially no gameplay value.
You can't even get classic cards from the arena anymore. I was lucky to have played a bit the month after launch so I had some of the classic cards already. For players who don't, there's practically no alternative to buying a huge bundle of classic packs as the amount of arena one would have to do in order to get the dust to make the classic collection via disenchanting GvG cards is outrageous, probably in the ballpark of an entire year of grinding arena.
GvG is relatively new, but what it has done to the barrier of entry is bizarre. It has essentially eliminated F2P for anyone who hasn't played all along, and raised the cost of starting Hearthstone by an absurd amount as you can't realistically obtain a classic collection without buying probably hundreds of packs.