The long leg is also a result of where your seatbones are positioned in the deepest part of the saddle. This differs quiet a lot from a jump type saddle (with a relatively shallow seat, and a dressage saddle, with a more pronounced deep seat). In a jump saddle, you don't have as obvious a 'home' position in the seat, because you wont' spend as much time IN it. You'll be up and in two point, or doing a standing ride, off your knee and foot.
in Dressage, you will be IN the seat almost entirely (depending on your level), so the 'home' is very clear. By 'home' (my own term), I mean the place where your seatbones will be naturally most balanced. you will just sort of 'sink' into the position, as long as the saddle is not uphill or downhill on the horses back due to being ill fitted .
The thigh blocks are meant to help you, the rider, keep your legs back, once you are seated down into the 'home' position. And, well, I agree that it can't really work well for all shapes and sizes.
Also, people think that a 'long leg' means to just have their thigh so vertical, and the stirrup so far down, that they are actually WORSE positioned. It makes them end up fishing for their stirrups, and often either leaning way back and balancing off the rein, or collapsing forward off of pinched thighs.
Long legs means that the weight of the rider flows downward, through the thigh, knee, ankle and ball of foot. No point is pinching, but all points have some engagement, of muscle and weight bearing. Over straightening the leg will not achieve this, and thigh blocks that force that position , before a rider is ready to have a straight leg, only cause tension.