At what point in a project is it considered beyond the "bootstrap" phase? The way I see it, Bootstrap is the perfect CSS template to use when starting a project. But at some point the project will reach a level of maturity and complexity where "upgrading" to a new version of Bootstrap is perhaps far more effort (and error-prone) than it's worth. This is especially true when they do things like renaming classes or reorganizing the layout structure.

Note I think this applies primarily to the CSS part of Bootstrap which I consider to be more of a template than a library or framework; the JavaScript plugins are always pretty painless to upgrade.