The once mighty refers to S1 5* heroes un-emblemed and un-costumed.
With 4* S3 heroes and costumed heroes fully emblemed they become stronger than your S1 5* heroes. I do not want to compare to which one is best as I believe it is unfair to compare emblemed and non emblemed heroes.
For me the once mighty has fallen (assuming they are without emblems and not in your defense team)
Here are a couple of reasons why:
A 5* hero (S1) no emblems are basically limited to your war bench.
The time and amount of emblems it takes to put 18 nodes on a 5* hero where you can do that to 3 4* heroes
Ascension materials needed to level them.
Difficulty in acquiring them.
As an example Mist’s special (much more effective) VS Richard’s special skill.
Other uses in the game:
Very limited whereas the 4* heroes gives you better options in depth and strength to compete in raid tournaments.
To support my argument
Are S1 heroes worth leveling if they will only be in your war squadron or have the once mighty heroes become totally irrelevant?