>Hard to know why Microsoft stopped developing IE6 and you may be right.
Surprisingly little known story: in short MS was afraid of Linux as desktop OS, OSS world was seen as capably of quickly copying any innovative single feature. This is resulted in idea of "integrated innovation": produce innovation in app development, app framework, data access, communication, os based on it and ship it all at once, so Linux would never catch up (.Net Framework, Avalon - WPF, WinFS, Indigo - WCF, Longhorn). Obviously, plan failed (well, Linux failed to come to desktop too).
So IE was seen as done and future to be elsewhere. After IE6 ship with XP, IE team was transformed into Avalon team, consumed bunch of other teams and at it's peak Avalon project had something like 500 people working on it (after much struggle and numerous delays it shipped as WPF). So for something like two years, starting at summer of 2001, only 4-6 people worked on IE - IE Sustained Engineering team, patching security vulnerabilities. Then it became obvious that they can't keep up and some rearchitecture and refactoring are required in order to fix security problems for real. So IE Security Team was created, and some old timers were transferred from Avalon to do this security fix up, probably around 20 people.
Then it became obvious that "integrated innovation" is, in fact, too integrated to ship on time, Avalon and Longhorn are nowhere is sight, and browser war is effectively being lost for the second time to Firefox, of all things (MS management couldn't take fork of OSS fork of NN seriously for awhile). So IE Security Team was expanded, became IE7 team and happily started to work on a browser again, 3-4 years too late.
>However, the next browser was scheduled to appear with the next OS, which was Vista, which turned into a debacle.
As you can see from the story above, initially it's wasn't scheduled to appear at all.