Just on the point about upgrading, I can remember between two versions of Bootstrap where markup changes were required in order for the site to function properl

Just on the point about upgrading, I can remember between two versions of Bootstrap where markup changes were required in order for the site to function properly with the new release, and no backwards compatibility was provided. Unless somebody is waiting for new features in the latest release, I can see them not wanting to bother going through and updating all of their forms or views. However, if the new features are compelling, it will be worth the effort. Really this is the same decision process we go through when deciding whether or not to upgrade any underlying framework or supporting software.