M$ is motivated by money, pure and simple. The only way they'd go back to the 98 codebase is either if they thought it would be more profitable to do so, or conversely, less profitable *not* to do so.
I think the only ways to do that would either show that the Win98 codebase will bring in more revenue (of course, since they can right now parlay the non-backwards-compatibility of WinXP into increased software sales and forced upgrades for them, good luck on that) or to create enough negative public opinion and people willing to switch away from Windows to make them pause (good luck on that too... most people aren't saavy enough to learn a new OS, not to mention how to handle all the compatibility problems in terms of drivers, programs, files, etc, so even if they hate Windows, most of them are stuck with it.).
Peace & Luv, Liz