There is really no right response to this, while you are correct most of those talents are obsolete...Now.  But that does not mean IGG won't do something to make them relevant again, so if you sell them all you will be behind the 8 ball.

On the other hand many members combined as many as they could and converted them to insignias.  In fact you can convert at any level, so maybe you want to convert some and sell the rest.  Regardless as to what advice you may receive, it is your account so ultimately it is your decision.  Choose wisely & Good Luck!

I would say it all depend on what is lvl of Your crests, do You have enough insignias, which can cover heroes You are using, and if those insignias are what you really want. There are crests which are still good like Slowdown (for specific scenarios) or Revitalize but rest is not quite that important especially if you have them on lvl up to 4. Cost of lvling up is really high, and i would say heroes need those resources more than crests. 
Personally I sold those i am not using long time ago, and selling more now will be waste of blue crystals. I am using crests as Blue Crystals storage, cause i am close to max, and there is no way to convert blue crystals to boxes. But if that will happen, i think i will sell what i have left to free warehouse space. 
So telling that if you have good insignias, not many blue crystals than sell crests from now on.