You don't think that this change will make old cards worthless.
For Example the Grum Flamblade, IMO you can use him only with Reverse Speed because what help you bloodlust if he don't hit ? Now everybody can take away Bloolust with Dispel and in Reverse Speed he get even worser hit chances every time he hit.

The best example how to make cards worthless is Malric Inferno and Delwyn dragonscale.
In case of Malric you place a new summoner with same +1 melee and addditonal +1 life for only 1 Mana more, I cannot imagine when it is not a good invest of 1 Mana for +1 Life for All friendly monster.

With Delwyn they bring out Archmage who add +1 Melee and +1 Range for only 1 Mana and cause this take not away the complete worth of delwyn they decreas Mana-cost of Archmage from 6 to 5 so that he cost same mana and addd + 1 Melee an + 1 Range "for free".

Why you should invest in a game when it seems that the goal of the team is to spoil your investment after you have spent your money ?