People are giving evidence. The argument hussar describes is more complocated than "everything from 4e is bad because 4e failed". Normally when people suggest 4e mechanics should make it into core 5E, they are mechanics which were related to 4e's unpopularity (surges, martial dailies or encounters, etc). The point of 5e is to re-unite the base but you cant do that if the very things thag caused the split are core parts of the new edition.
Do you deny that 4e powers or healing surges were divisive innovations? Because those are among the chief criticisms of 4e.
You're right, of course. What Hussar is saying is that the mere fact that they are from 4E should not ban them from making it to 5E. However, the mechanic itself can be attacked.
I do NOT want healing surges in the next edition. I dislike their very premise. If I hit you with a sword for more than 1 HP, NOTHING except time/medical attention or magic will get you back in shape and ONLY magic will get you back in shape within a day. PERIOD.
Now, what did I do there? I explained why I, personally don't like the mechanic and what the problem I see with the mechanic is. There fact that it belongs to 4E is besides the point.
However, since all arguments have two sides, the question really comes down to "should this mechanic be retained?" and THAT depends on several factors: Is the mechanic sound? Is it good etc.
An extra point where the fact it is from 4E DOES come in is the point Bedrockgames raises above. IS the mechanic divisive?
THIS is where 4E should be taken into consideration. IMO, 4E was far, far more divisive than any previous edition and that stigma must be taken into account. If a given mechanic cannot be included in 5E in such a way that it call be removed or replaced easily (ie it is a basic premise of the entire edition), then its 'divisiveness' is a MAJOR factor since the stated aim of 5E is to reunite the editions.
As of now, I am aware of no mechanic that fits this description because we have, to date, no real information on what mechanics are being brought into 5E from ANY edition.
So we should wait and see. In the meantime, the basic point remains: a mechanic should not be excluded simply because it is from 4E.