The better the type system is, the more bugs it prevents, so one has to spend less time unit testing. Sadly, it also means the language is harder to learn, so the rest of us are stuck with horrible, unsafe languages.
I actually think there is literally no downside to making typing opt out rather than opt in - beginners benefit from the compiler saying they accidentally swapped two arguments around - how could they not?
I have helped beginners with untyped Python codebases and it's a lot harder to tell what function parameters are supposed to be than in Java or C++, leaving aside any other considerations.
Mentally parsing the type errors is an art though - generally you want to ignore pretty much all but the bottom 1 or 2 lines - the rest is a bunch of very verbose information at a much higher level of abstraction than you care about.