The argument against testing seems to be "the Spike", when you're learning a language/concept/framework or are testing out ideas. But startups (at least the two I've worked for the last two years) seem way more heavily invested in the spike than in the carefully-tested production environment. Personally speaking, it's even more exciting to work on.

It took me a good year and half to properly appreciate well-optimized, well-tested code in production, and enjoy the challenges associated there (which aren't trivial-- the neatest, ugliest NumPy script can be entirely useless at scale, or in a timeframe useful to users).

But I still personally feel flat when the knowledge gains associated with a Spike aren't as frequent. How have other developers and math-types dealt with that disappointing feeling of a plateau? Besides just changing jobs entirely, I mean.