We need to remember that XP support was extended for some five years, it's still active 'til 2014 concerning security issues. The main issue is probably the one that was with both Windows 98 and XP: corporate customers. When it comes to business, it's about $$, and the amounts are higher / more reliable than those in consumer part. Consumers are actually food, eaten by big-enough companies.
Windows 98 and XP were not extended by popular demand. They were extended by [Big] company demand. Most probably Windows 7 will, likewise, be extended. The simple rule of money: Companies will not pay.