So I also love Tcl, but I think it would have not been great as a web language. I think the small amount of type information we can pass via json is just about right, and tcl would have made it impossible.

You cannot encode arbitrary tcl data to a typed object "correctly" because every tcl dictionary is a valid list, and every list is a valid string (numbers are also strings, and so is code). So in a world of "tcl code on the web", both the sending and parsing ends of a 'tcl data stream' would need to know which type they're "supposed" to treat the data as.