Removing DFG from the game was a terrible decision with a bullshit excuse. Rito claimed that it was a must-buy that made assassins 100 - 0 squishies. First off, DFG was a far less common buy than Rito propagandized the bleating masses into believing. Second, assassins didn't need this to 1-shot squishies, they're fucking assassins, they did that on their own.
What DFG did in fact give mages, was an answer to tanks and bruisers who built multiple MR and HP items, this is the main situation the item was being built into. When they removed DFG from the game, it left a hole in the item choices for an answer in dealing with beefy enemies. Rito puclicly admitted this problem and PROMISED they would give us a replacement. It's been more than 2 years now and they still haven't delivered on that promise.
Instead of a replacement and Rito admitted it wasn't a replacement, they gave us Luden's Echo. This item is far more toxic and has been far more difficult to balance than DFG ever was. While DFG wasn't suitable for all AP champs, it was only viable on characters that could dump there whole combo within its timeframe. Echo on the other hand can be built on literally anyone, gives a gut-wrenchingly obscene +10% MS and creates an artificial damage source. Artificial damage has proven to be one of the most imbalanced tools in this game's history, hence the repeated nerfs to EVERY Spellblade item, the nightmare that was pentakill-whoever-the-fuck-you-pick Runeglaive and the various other nerfs to Energized, Echo and every other artificial damage passive. The problem with artificial damage is that it ignores a champion's kit and thus allows even low-damage characters to suddenly begin outputting real damage that they weren't designed to do. Which unfortunately most players and Ritoers ignore these facts when looking at data that is directly affected and artificially increased because.