Spokes have tensile strength of approx 1100 MPa.
Typical spoke diameter is 2 mm.
Lets assume you have 2 wheels both with 36 spokes. On each wheel 18 spokes act as "braking spokes" due to the spoke pattern. In total you have 36 "braking spokes".
Main formula: p=F/A
where p=pressure=tensile strength
A=Cross sectional area
Lets calculate the cross sectional area
With a total of 36 braking spokes you have 3454*36=124344 N
which equals 12,6 tonnes.
Now, unless you are able to stop your bike literally within a fraction of a second (=enormous decelleration) I hereby claim that you will never reach the 12,6 tonnes limit. Now, one can extend the calculus with all different kinds of variables, but this was only to show how strong spokes actually are.
As a comparison:
Construction steel tensile strength: 370 MPa
And the beforementioned stainless steel spoke: 1100 MPa
... and even higher for thinner spokes.
Thus, spoke material is 3 times stronger than construction steel.
However, if your spokes have experienced extensive fatigue due to bad/wrong wheel building, a large strain like a downhill brake might just be what is required for a snap. But thats a different story and disc brakes cannot be accused for that matter.
Now, this doesn't mean I run disc brakes myself. I have used disc brakes and they are fun in dry sunny weather in town. I only run hydraulic rim brakes (=Magura). My Maguras have not required any service or a drop of oil for the last 13 years.