I didn’t say I want to track purity 
That’s probably too much. Writing to mutable state? Probably not. Performing a network call? Probably yes. Maybe tracking non-local computation would be a good, precise term?
(in fact you propose the same in the next section, as I now see)
Java already makes them identical via various RPC frameworks.
Not always - you often get different checked exceptions, which is a way of “marking” a method as side-effecting. Where we have IO[], java often has throws IOException - both influence the signatures. But again given history, we might be looking for better solutions than checked exceptions (I think in general in Scala we are looking for better solutions to various problems 
).
Do we want the compiler to point out that we might not be handling all the error cases that we should (which could lead to applying e.g. a retry/timeout strategy)? I think in typed a language the answer might be “yes”.
I do not think that question will ever have agreement, which maybe argues Scala should be more opinionated so as to select for a user base compatible with its goals.
There definitely won’t be agreement, but luckily we have EPFL and Martin who picks the direction as the where Scala should be headed (with input from the community of course, but ultimately somebody has to make some choices from time to time).