le Pingouin,
The other thing that occurred to me is that whilst Microsoft may well be extending support for XP a little longer, one should not forget about third party software developers and hardware manufacturers.
You may find that they increasingly fail to support Microsoft's ageing versions, either by not compiling software/drivers for the old versions at all, or putting the bugs for the old versions on the back-burner to be sorted out at leisure.
Overall you're right though, the incremental cost to Microsoft of pushing updates for Home versions is probably minimal as 90% of the code is shared and 10% is features, the code for which is probably pretty stable anyway as it's to do with important business stuff like AD etc.... so most bugs there have probably already been ironed out by now. Its the main codebase that's the ongoing worry.