Most of my time isn't academic or writing code. Most of it is focused on enhancing algorithms to achieve what's required for new methods to work deployed in production environments such as the Pumas clinical pharmacology work, and now working on what's required to get scientific machine learning point-and-click GUI ready. I always leave a little bit of time in mornings and weekends though for some coding, and a little bit a day goes a long way after years.

I stay away from code rewrites like the plague, which is why SciML maintains so many wrapper packages. It's less work and if there's nothing new then rewriting is thankless work. Unless we have some kind of angle for something new in the algorithm, we just recommend someone use the same old wrappers (for instance, IDA with fully-implicit DAEs). We won't beat the C/Fortran code without something new. However in most areas there's lots of interesting research to be done. I could "sabbatical as an academic" and spend the next 3 years just improving explicit Runge-Kutta methods: those for example are probably still 4x away or so from what I think is theoretically possible, but that's not going to happen any time soon since we have some real applications to focus on.

Those package deprecations mentioned above were deprecated by community efforts (BifurcationKit.jl replaced PyDSTool wrappers and was done by Romain Veltz, FEniCS wrappers were largely replaced by Gridap, and SymEngine was replaced by Symbolics which is largely the work of one of the Julia Lab PhD students Shashi). Even those had a largely new element to them though, with BifurcationKit focusing a lot on recent algorithms that other bifurcation software don't have (like the deflated Newton methods), and Shashi's Symbolics.jl work focusing on generality of term rewriting systems to allow for alternative algebras.