My high school physics understanding of reinforced concrete is that the coefficients of expansion for steel and concrete are similar to a degree that it is very resilient to heat stress. I doubt that's the case for silk.

The problem for such an application is not the strength, but the chemical stability in time.

For it to be competitive, it would be necessary for the rate of hydrolysis of the protein fibers (which breaks the fibers) in an alkaline environment to be less than the rate at which rusting diminishes the strength of steel bars.

This is very unlikely. At most there might be a chance for such fibers to be used for reinforcing some other kind of cement, less alkaline than the common cements.

Also, for reinforcement applications, good adhesion between the reinforcing material and the reinforced material is necessary, which for silk vs. a mineral cement would need some kind of extra adhesive coating of the fibers, with adequate properties, which might be hard to discover.

Elastic stretching, the one that reverses completely on taking the load off, is the regime where there is a linear relationship between the stress load and the stretching. This should be seen as different from yielding which in mechanical testing is an irreversible change usually signifying a destructive change in the material. So, yes, silk stretches but is not damaged while doing so and the resistance it provides at the time relates to its strength.

Apparently Dyneema has a tensile strength of 3.5Gpa

https://matweb.com/search/datasheet.aspx?matguid=4481722d60e...