We had a C application server and used TCL as a business logic language. It was a great fit for writing code fast and ease of maintenance.
You can hear me get a laugh when I say we use TCL in a talk I did here:
Worse yet, a lot of the tools aren't open source or freely available, even for tinkering. I'll happily pay for a license if I end up using it for business, but I'm not going to pay hundreds of dollars up front just to play around with a niche language.
There's lots of great languages out there which include completely free and open source tools. Maybe I'm spoiled.
I can't think of how Tcl/Tk docs can be massively outdated given that Tcl/Tk maintains almost 100% backwards compatibility, so most of the old advice works just as is.
Regarding open source, nearly all tools are not only open source, but usually under very permissive licenses (e.g. Expat or BSD-2), and there are in fact very few commercial closed-source tools (basically Komodo, what else?).
Are you sure you actually tried Tcl/Tk and not something else?
As an aside, Tcl was influential as a source of inspiration in the early days, ranging from event-driven programming, to web servers big (AOLserver) and small (Brent Welch and Stephen Uhler's <200 line server/HTML parser) back when "normal" was forked CGI scripts.