It's rational to be motivated entirely by the net present value of one's future earnings.

There might be other relevant sources of utility for the developer, but did you really want me to complicate the post by discussing them?

Programmers are immortal (and never switch career.)

I think your discount rate is supposed to take the probability of your dying each year into account. Ditto the above regarding career switching/retirement.

A programmer's skill increases in a smooth, deterministic, predictable and exponential manner over time.

Not necessary. For my model to be useful, you just need to know the expected rate at which the programmer's wealth-creation ability will improve. The formula gives the programmer's expected rate of instantaneous value creation.

A programmer's skill is a scalar-valued quantity proportional to their (potential) income.

Taboo skill.

Anyway, the real question is whether using this model or some approximation of it produces better results than relying on one's native intuition. Relevant link.