I am at the point where I can do consist pistol squats weighted. It seems to me that pistol squats are much safer than heavy weight back squats. Can pistol squats be a replacement for back-squats?
They're fundamentally different exercises.
If by "replace" you mean:
Can a person do only weighted pistol squats instead of barbell back squats and live a long and healthy life?
Than the answer is sure. But if your question is:
Can a person do only weighted pistol squats instead of barbell back squats and get the same strength and mass gains?
The answer is no.
From a purely functional perspective, I think the pistol squat really wins out. It requires much more balance and coordination, and forces you to really use both sides of your body. I'm sure you found weaknesses and weird little quirks resulting from various imbalances.
But strength and muscle tissue is not to be disregarded. A properly performed barbell back squat strengthens the lower back, abdominals, and really everything from the bottom of your rib cage down to your feet. Further, progression is much easier and more manageable and you can train for substantially more power production than would be safe with a pistol squat.
Additionally, the pistol squat is an advanced exercise. Like ab wheel rollouts, they're not an exercise anyone's introducing to a new trainee as where air squats or bare-bar squats are certainly on deck. Barbell squats also teach you a neutral spinal position for practical issues like moving a couch and for other training moves like the deadlift.
I generally only do standing overhead press, usually avoiding the benchpress. One could correctly argue that I'm losing a bit by not benching, but I can press 5x at 135lbs which I think is plenty enough chest and shoulder strength for me. Weighted (and even unweighted) pistols easily clear the bar of "good fitness". If you have more specific goals, it might not.