Steel decays via rusting as its outer coating is sacrificed to corrosion. Civil features decay as erosion degrades it over time. Wooden power poles decay as their treatment degrades and fungi/insects attack them. Outdoor wiring decays if in direct sunlight due to any sunlight resistive coating degrading over time to UV radiation. Oil used as lubricant in motor vehicles and as insulating fluid in electrical equipment degrades over time due to thermal cycling, oxidation, and moisture.

The point I'm making is that things degrade naturally. Plastic is no exception, although engineers have been able to make certain decisions with it such that constructions can last for decades.

If we can make plastic by default biodegrade naturally, and at a much faster time scale than today's oxo-degradable and biodegradable alternatives, then it still allows for scientists and engineers to select for plastics that have been specifically engineered for the application via coatings and whatnot, comparable to steel and wood.

It's possible to do so. We just need to flip the script and make biodegradation the norm and not the exception

Yeah, for things that are supposed to endure, biodegradability is indeed problematic. However, using plastics for things such as wiring insulation would be still a potential source of microplastics even in a world where all plastic was abandoned in favor of fungi and paper packing materials. Ain't no easy solution, unfortunately.