We shouldn't be saying that NPCs in general need to be harder.
Because there are already hard NPCs out there, if you go looking for them. Conflict zones, pirate lord assassination missions, pirate activity detected signals, etc.
What we should be asking for is more variation and a smarter application of this diversity of difficulty in appropriate places.
Right now, I am ranked "Dangerous" and I never need to worry about NPCs. There isn't a single situation that NPCs concern me. I frequently fly around with hundreds of millions worth of cargo, but the NPCs I encounter, even the hostile ones, are no challenge. My opinion is that interdictions by Deadly or Elite NPCs should be as difficult to evade as a player interdiction. I often feel like I'm cheating when I evade a Deadly Mamba in my Cutter - a behemoth of a ship. I almost want to lose just so things felt a little dangerous.
I don't want noobs to be shredded though...so lower ranked ones should continue to be easier.
The point is, the further you progress..whether it's your combat rank, or the value of the cargo you are carrying...the game should be more challenging and risky. NPCs in general span across all difficulties. You just never get the hard ones unless you specifically go looking for it...even if you're highly ranked and carrying 400 LTDs.
People complain about mining being so OP, but we always focus on the payouts. The payouts aren't the problem. The problem is that "Mostly Harmless" Cutters can fly around with no shields, carrying tons of cargo, and never have to worry about a challenge, or if they can evade NPCs.