I am opposite in this case. Wristblades should be and look like they are compactable/retractable. Anything bigger does not make sense to be attached to a forearm/wrist.
Even if looking at it from realistic point of view, it is harder to maneuver long blades or blade attached this way. Not to mention a direct attack must be less powerful in comparison to shorter wristblades or short sword based on physics.
Well following how the first Predator movie does it, it feels part well of how the wristblades extend and retract.
Hell even they could have had a retracted position similar to how the AVP wristblades do it like in this here concept art: First one displayed
I'm not going to lie though, I always found the longer versions of the AVP ones to look a bit silly.
As for an realistic point of view, not only that Preds are much stronger and faster than us so their power output into these blades would make for some nasty killing blows, but also there are real life related weapon from India known as the Katar and its incredibly effective and deadly weapon that the people of India have used for centuries.
Slightly off topic; it would be pretty cool to see some Indian martial arts based around the Katar implemented in Predator wristblade choreography