If they will be parallel line (base tactical box with future upgrade sets for assault and heavy weapons, new special vehicles, characters etc.) then I am ok with it. Sort of.
From fluff perspective they won't be sub-faction so how they will be incorporated to existing chapters? Will they replace company captains and chapter masters as they will be superior to them in every way? Will all codex and non-codex chapters have them ?
If they would be just small elite addition because of production cost, then it would be ok. But it they want to bring back entire chapters this might not be the case then.
I think that sooner or later old models will be replaced completely. Slow version of soon-to-be End Times is coming