That's true of several people... Blackrock got utterly RUINED.
Pre-CoIE:
lesser villain, but unique enough....
New Earth:
Ok, this was conceptually the same power source, but used in fundamentally different ways.... and passed to several people.
New 52:
Wut? not even vaguely similar.
I don't know why you feel the way you do, since there is no evidence that DC is even taking that route. The "everything is canon" thing simply means that past stories are still relevant. That's it. We're still post-Nu52/whatever iteration. And quite frankly, I rather read the stories on their own merit, rather than worry about where in the timeline an event occurs. Besides, if the storyline is popular, it will keep. If not, it will soon be forgotten. No more, no less.
I'd rather not be guessing if we're using Supergirls post COIE origin or New52 origin. Or even Pre-COIE origin.
Is Cassie, Zeuses daughter or not?
Does the Geoff Jones run on TT still valid?
I like history with my characters and being able to see how they grew or what they've been through.
There's been a blurring of lines between all of these.
If you don't care, more power to you.
The "Everything is canon" concept is incredibly stupid. Considering who wanted to implement it makes it unsurprising, however.
The Multiverse is the solution and always has been. COIE could have more easily switched the focus to a new universe and Earth instead of destroying the infinite Multiverse. It looks like DC is realizing this with the rumored Absolute line, though I have a feeling they'll find a way to botch it. I'm somewhat hopeful, though. It's way past time to move on from the post-COIE DCU and its variations.
Keep in mind that you have about as much chance of changing my mind as I do of changing yours.
The problem is just there are some cases where it would be for the relation between certain characters pretty important to know if certain things are canon aren't.
I think it was for example pretty wired when they did the post rebirth young justice series, without clarifying if the Teen Titan runs these characters have been in together after that were also in canon, since the series was pretty much written like neither the pre flashpoint nor the new52 Teen titans run had happend.
And than you had stuff like Tim Drakes parents where we had several years of Tim staring in comics where it was completely unclear if the were alive or dead.
And than you have imo also occasionally cases where they make in one story call backs to stuff from previous runs, even if what happend in those runs completely contradicts the current status quo.
This simply isn't true. The post-Superman Reborn, post-Death Metal, post-Dark Crisis "continuity" (if such a thing exists now) does indeed appear to be a completely distinct beast from both pre-Flashpoint and New 52 continuities. You can't honestly say it's just New 52 with minor changes.