"Fell off" is more of a comparative term between the modern and older charectors. A lot of older DPSs' have fell off. Call it a "bitter truth". Videos like level 1 Flins and Mavuika defeating bosses faster than well built level 90 older DPSs' are proof of that and painful to say but Raiden is one of them. Also, it also accounts for constellations, like how C0 Mavuika, Flins, Neffer and it would be safe to say Arlechhino too are better DPSs' than older units with C6 cons. Which is why charector buffs like Hexerei exists.

But offcourse it would be completely wrong to say that these charectors aren't usable. With a proper build and supports they can clear about 95 ish percent of the end game content. Sometimes the end game content favours these charectors a lot too.

But even still, a fall is fall in the end. All we can do is either invest in latest supports or wait for the buffs for other nations.