Although at fist glance this formula is written recursively, one doesn't have to (and shouldn't) implement using recursion, does one? Just making f, g, q, d arrays and then loop over t should be good, or is there more to this formula?

Appreciate the curiosity. In this small trivial case, yes that works. But what happens when something in the logic changes?

You wind up needing to know the order of calculations since things are no longer lazily evaluated via recursion. This is a problem when you have dozens of "columns" (i.e. recursive functions or arrays as you are suggesting). Often times, the value in the array is NULL (or worse, leftover from a previous calculation). You are left to manually try and re-order the calculations, which is not trivial when there are hundreds of functions.

Excel takes care of these details for you automatically. Users program functionally and recursively (fill-down) without even thinking about it. Excel reactively updates when dependent values change (re-evaluates as necessary).

If power, speed, and scale are necessary, there are purpose-built systems (with Domain Specific Languages) which specifically solve this problem in the insurance domain (e.g. FIS Prophet, Risk Agility, AXIS, etc).

It is common knowledge that all recursive functions can be re-written using iteration (e.g. loops). See “ Recursion versus iteration” here https://en.m.wikipedia.org/wiki/Recursion_(computer_science). The assumption that only trivial calculations can occur using iteration, or that recursion alone allows for supportable code, I believe are very flawed assumptions.