There is of course a point of diminished returns when it comes to price, but a good and well designed light weight bike handles better than an overweight bike and having more gears to choose from is more efficient and allows you to optimize you cadence. It is especially important if you end up having to pedal home unassisted.
Just because two bikes look similar does not make them equal. I think that the bikes that Harley offers are quite overpriced, but I doubt that your girlfriend got an equal bike for $800.00
You say that you have had this bike for three years. That in and of itself says nothing about durability. How many miles have you put on it in those three years? That’s what matters.
If you are happy with a 70 pound bike with 7 speeds, then that is good for you and there is no reason for you to pay more. There are others that desire a broader gear range and want better handling, smoother shifting and better braking. For many, the ebike is a primary form of transportation or recreation and if they can afford and want a lighter and more efficient bike, then why wouldn’t they pay more and gcet what they want? That doesn’t make them, as you said, suckers.