For me the only confusion is:
Why you expect Norton to predict the effect of Microsoft ceasing to supply security updates for Microsft products. Norton deals with attacks not with the holes in other people's products. Norton have indicated their policy on this.
You refer to XP as if it were one product which it is not. Go to Microsoft and read what it says about its own policy:
Here is a link to the table of
Microsoft Support LifecycleProduct support lifecycle information by product family
Windows XP Home Edition
Windows XP Media Center Edition 2002
Windows XP Media Center Edition 2004
Windows XP Media Center Edition 2005
Windows XP Professional
Windows XP Professional x64 Edition
The table at that location gives the dates. I would have pasted it in here but the way it is formated for their web page makes this almost impossible but you will find that it is quite different for Home and Business versions -- go have a look.
And the FAQs that they link to include:
17. What is the Security Update policy?
Microsoft has not changed its Security Update policy:
Business and Development software
Security updates will be available through the end of the Extended Support phase (five years of Mainstream Support plus five years of the Extended Support) at no additional cost for most products. Security updates will be posted on the Microsoft Update Web site during both the Mainstream and the Extended Support phase.
Consumer, Hardware, and Multimedia products
Security updates will be available through the end of the Mainstream Support phase.