In addition to what LF has said, DFG was designed as a "tank killer" item back in the days when FoN was a ubiquitous purchase on all front liners. DFG was pretty good at being a tank buster. But it was the ultimate squishy killer allowing numerous champs to have a one shot play style as well as a purchaseable ranged point and click execute.
Imagine if blue smite didn't slow, had 50% more range, and dealt 10x to 50x the damage. It'd be a legitimate purchase for any champ at any time. And a must purchase for some.
And then on top of this the item was rather gold efficient as well.
The very existence of the item meant tanks had to be made even more tanky to be a tank and that adcs were largely useless until extremely late game when they could itemize a defensive item like GA plus games would be extremely snowbally which made late games rare to reach.