There's enough competition in the web browser market to prevent a lock-in in IE6 scale. There's IE itself, still a massive force in enterprise, there's Firefox, there's Apple with its own Webkit fork in OSX/iOS...

edit: also, these days browsers work together in standardization communities and implement stuff from other browsers. This was basically non-existent at IE6 time, in addition to IE6 being basically frozen and closed for years.