Blobs of glass are actually overwhelmingly linear :) Non-linear effects in optics are generally negligible unless you go to very high energies (scattering, like raman scattering) or special materials (like fluorescent ones). You need a high powered laser or certain materials for those regimes.

That said, the geometric functions (i.e. the positions of rays w.r.t. other objects) are probably non-linear in the object position parameters indeed, and intensities certainly are (example: moving an object in front of a screen by X amount changes the illumination in front of the screen non-linearly): it's important to keep in mind what we mean by linearity (it's in terms of light field intensities for a fixed geometry scene) -- the scene are the transfer function coefficients.

What really makes things complicated in real life is (1) the presence of noise; (2) imperfections (and unknowns) in your physical/geometric apparatus. Even if you know the system perfectly, noise generally disallows reverting (or easily reverting) transfer functions, i.e. undoing blurs and arbitrarily refocusing images with simple detectors. The imperfections and even lack of rigidity of lenses and your system add even more difficulty. That's why making a simple computational lens is not so easy.

It sure does. But if you can replace many of the lenses with computation it's a big win. If you can compensate for some of the inherent aberration in a lens design, or specific manufacturing variation in a lens, its an even bigger win.

In other words, one does not obviate the other.

That is actually deliberate, we tend to organize everyday life and the devices we build around processes that are easy to model, that is why many things look like linear processes and harmonic oscillators (the first two term of the Taylor expansion of the actual behavior). We change the type of spring we use if the current one starts to wear out early under normal operational conditions.

I think it’s more that we developed linear algebra that made us approximate physical processes by linear models. If you look close enough, almost nothing is linear.

For example, we happily draw a linear scale on a mercury thermometer, even though we know that not to be correct, even if we incorrectly assume that the coefficient of expansion of mercury is independent of its temperature. Also, try explaining why the conversions between Celsius, Fahrenheit and Kelvin are all linear (answer for Kelvin and Celsius: they technically only are since 2019, when we redefined them (https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba...). I think the Fahrenheit scale was redefined as a shift on the Kelvin one around the same time)