Does the pilot still have final control authority on modern Boeing airplanes?
I think you fell for a common misunderstanding:

Fly by wire ≠ flight control augmentation 

It is - in the first place - another method to get the pilots input out to the control surface just like cables or pushrods. Just not mechanical but electric/digital. 

If you want to alter the pilot input for whatever reason it is way easier with digital signals then with a mechanical input. 

Example: The speed brake in an A320 vs in a B747. If you rapidly extend the speed brake in an A320 aircraft, the extension rate is limited by a computer making for a smooth movement of the control surface. If you do the same in a B747 the system tries to match the lever position as fast as possible with all available hydraulic power (3000 psi). So rip on the speed brake lever in a B747 and the heavy aircraft will jolt quite a bit.

Concerning flight control augmentation it really boils down to the design philosophy. In a B747 you can do basically whatever you want (even stalling) but in the B747-8 you have some systems augmenting pilots input. Tail strike prevention on rotation and the nose down trim when flying a flaps 30 approach come to mind. I can't speak for the B787 but the B747-8 is, despite its largely fly-by-wire architecture, still a conventional airplane with (almost) full control authority by the pilots.

The B777 family brings flight control augmentation to the table where the computer shows you the limits via the control forces generated in the control column. The computer shows you where it thinks you should stop but if you decide to press on you just have to double the force exerted on the control column and the aircraft will do what you want.

The Skybrary has a good primer article on flight control laws for both Airbus and Boeing designs. It gives a good overview about the similarities and the differences in design and philosophy. Shoutout to user Simon Opit who brought it up.