Often, projects use programming language X, but would use programming language Y if they were started from scratch.
For example, big numerical models may be written entirely in Fortran. Whereas this might be a reasonable choice for the components that need to run fast (alternative would be C or C++), it might be a poor choice for components that either do not need to run fast (such as things dealing with human input or simple visualisations), or where runtime is not the limiting factor (such as I/O, particularly when from the network).
Another example may be when a project is built using a propriety language (such as Matlab; no, FOSS clones are not good enough) and was started at a time when FOSS alternatives were not viable, but ten years later, they are; and it would be beneficial to migrate.
However, due to language inertia, a migration does not happen. Code that works should not be touched, porting code is a time-consuming, expensive process, and programmers are familiar in language X but not necessarily in language Y. Still, in the long term, a migration would likely be beneficial.
Can anything be done to mitigate the problems associated with language inertia? Are there any notable examples of big projects that have successfully overcome this problem? Or is a project bound to stick forever with the initial choices?