[eertami]

>Check newest MS Word you can still open Word 97 documents.

It's not really a fair comparison at all. One is a stylesheet, the other is a desktop office application.

>but until now its been a normal thing that the new version is compatible with old one

Not true at all, look at Python 2->3, PHP 4->5, jQuery 1.x->2.x. Major releases often break backwards compatibility.

>from spending hours and hours rewriting their codebase to match your newest release

Design is not something you have to rewrite to 'match the newest release', there are no security vulnerabilities in a style sheet. It is fine to stay in Bootstrap 2.