Hearthpwn

https://www.hearthpwn.com/news/6437-mike-donais-on-hall-of-fame-malygos-mind-blast

There is a very simple answer to OP's question:

The game needs combo tools in order to keep the meta from stagnating into the most plodding control vs control abomination you can imagine.  It is possible to slant a control deck so hard against aggro such that it will win the matchup 80-90% of the time.  Without a combo option that should beat those control decks quite easily, there would be no incentive to play anything else.

Despite what the folks who look down on aggro decks seem to believe, there is nothing attractive about playing this game in such a way that every single game ends ten turns into fatigue.

you can pry malygos from my cold dead hands I aint leaving em 

Quote from qb_maximus >>

Quote from Bluelights >>

I hate Velen Druid

 What about Velen Rogue?

HoFing them will require replacement. You cannot just take two more legendaries from the classic set and don’t replace them. Especially Velen, which is after all class specific legendary (despite TS playing against Velen Druid and Rogue)

 The difference is Velen Rogue is technically possible with a thief deck vs. priest.

I think it is wrong to say that these cards always find a way to be abused. There have been plenty of metas where these cards saw very little to no competitive play at all. They only pop up when broken cards are introduced that allow them to be abuse, and I think that shows that they are not the cards that are broken, it is the cards that allow them to be abused that are broken.

It would suck for blizz to remove all diversity. Keeping certain cards, keeps certain diversity. The current meta allows for more strategies to be in play. It's great actually. It's not like the cards are OP or anything. They do have potential though. Good call Blizzard.