Sell them for a billion dollars, make the obtained cards undustable, make deck recipes with 1 or 0 legendaries.

There would be unending complaining if any of these actually happened

It makes for a much better new player experience

No it doesn't, new players shouldn't get a netdeck to start out with. They should progressively build decks with their resources, and should learn to prioritize which cards are used more and should therefore be crafted

it's a great learning experience (you get to see how the deck plays, understand what's missing and progressively improve it)

That implies you would buy an unfinished, unoptimized deck which just means that you wasted resources buying a bad deck. This seems terrible and would definitely see complaints if actually done.

people will still buy packs, because the decks you sell won't be exactly tier 1 and can be missing whatever key cards you want

See previous point. People won't spend money on it if it is just bad.

Magic has been doing that for decades

Magic also has trading and selling, so cards aren't necessarily as obsolete. Also, magic doesn't have the same type of ladder/competitive system as hearthstone, so there isn't the same need for decks to be competitive to be played.