Hearthpwn

As Blizzard has previously made very clear, the classic set acts as the basis for most successful and enduring decks, for example Zoo Warlock or Control Mage. Most of the necessary cards to build the deck are found in the classic set, and since it doesn't rotate the decks will always be there in some capacity. Blizzard has nerfed classic and basic cards (or moved them to the hall of fame) since these long-enduring decks are always so prominent. Blizzard likes to have unique metas and some cards don't allow for that, however they still like to keep basic archetypes as part of a class's identity.

In  my opinion, Blizzard won't dare to make the classic set free as it will hurt their profits ,as a business, too much. If everyone gets the classic cards for free then there won't be much point in spending actual money on the game as you could just use in-game gold to buy packs and won't really need to anyways since you will have most of the key components for any deck and could just operate off of free packs from events. Also,  there is a lifetime quest to collect all of the classic cards so that would just hurt Blizzard's profit even more.