I'll state what RIMM stated.
They will use the BB7 phones to suit the needs of low end consumers while they phase it out in favor of lower cost BB10s.
While I agree, I think RIMM intends on bringing a premium BB10 to market this summer prior to getting a lower cost phone out. They need something to go head to head with the Samsung GS4. Period!
How to get lower cost phones though? You need to group features together like a car.
If the Z10 is a BMW 5 series, how do you create a 3 series and 1 series?
3 series: less memory, dual core but much slower processor. lower resolution camera.
1 series: same meory as 3 series but even slower dual core processors, maybe even go single core. Cameras even cheaper. Somehow getting lower resolution screens? No expandable memory? USB memory attachemnt disabled. Can't be used for corporate use.
Thing is, this all needs to be done at cheaper price points also. It's hard to take $150 of hardware out of a phone without turning it into hated junk. But more importantly, you need a reason for people to want hte higher priced phones. Angry Birds might suck on a 1 series for example (I know I hate later iterations of it on my Droid 3 due to lag)