Have you seen multiple kLOC tcl apps? I have, and it is not pretty :)
Thing about extension languages is that they tend to be nice languages to write code, so people write a lot of code in them, and suddenly your app is 95% written in the extension language. Scheme is the language I know that grows best from a simple extension language to a full-fledged mature programming language.
Tcl has been improved a lot since the Tcl War (17 years ago!) including namespaces, module/package system, a native OO, a sane thread support, and Dicts. I am not sure if it is still true that Tcl doesn't scale well.
By the way I wrote a toy, buggy and ugly scheme implementation in pure Tcl a year ago: http://wiki.tcl.tk/25512
If it's a large tcl "app", then quite often that's the first mistake. I've seen large Tcl systems which work pretty well. A combination of different applications, cron jobs, scripts etc., with some C/C++ libraries for integration and performance-critical things. With the added benefit of having a pretty decent graphical toolkit on top. Won't be the winner of a GUI beauty pageant, but it works.
And most of those systems were pretty old, so it's not like you had a lot of better options back in the days. Tcl was pretty awesome in the mid-90s.
Most Java programs that replaced those enterprise tcl systems were a lot worse than their predecessors.
