Maybe you mean how the devs designed (and did their own playtesting) the game? How they play it outside of work could be anyone's guess...that the devs play the game differently than many/most players.
Of course they are fairly popular as they increase your character's power and/or utility. I don't recall if they were optional in 3E, but I don't think they were? They were baked in and part of the design IIRC.They may not use feats personally, but they are included because they are generally popular.
Regardless, they are optional in 5E, some tables don't allow them (I've considered that myself TBH), so some of the analysis should include work without feats IMO.
I wasn't part of the design team, so I really can't say. I am not sure what you think is "nostalgia" about it, and the flat math part has been an issue with d20 D&D since 3E came out. Bounded accuracy is a different issue IMO.Let's just face it: 5e wasn't designed for balance. It was designed for nostalgia and flat math.