tcl lost its popularity because it was displaced. By Lua. Because Lua had a lower dissonance with C and C++ (actual data types & al, for instance).

Could lua itself be displaced? Sure, but there's no contender I see right now. And I don't think that's an other point of view.

from what i remember, tcl's popularity waned long before lua caught on in a big way. people were looking to ruby and python as extension languages, even though they had a far higher impedance mismatch than tcl, simply because they were vastly more popular as languages by then