Conventional hard-drives are expensive to manufacturer and don't get much cheaper over time, you can just produce a "larger" hard drive for a similar price. In contrast, Flash memory is usually very expensive at large memory sizes but is very inexpensive at small memory sizes.

As a result of this, Nintendo has favoured Flash memory in their system because it allows their system to steadily fall in manufacturing cost throughout the system's life; while Sony and Microsoft repeatedly released systems with larger discs at the same price because there were no cost savings in having smaller hard-drives. This is one of the reasons why the PS3 and XBox 360 are still (essentially) $300 with a hard-drive over 6 years after their systems launched.

I can't see Sony or Microsoft switching to a SSD because, while you can get more storage than a flash drive, they are far more expensive than a hard-drive with far less storage.

Technically, the Wii and Wii U have embedded flash memory not a SSD

I know but just to have the option there.  They should have a small amount of flash or SSD for the OS and apps, then an HDD for everything else, but leave a slot to expand the storage via an SSD.  The load time benefits would be outstanding and I would be willing to pay.  Hopefully SSDs come down in price the next couple years too so if I run out of space (since 512gb is the largest there seems to be without completely breaking the bank) I could pick one up for not quite as much down the road.

I bet it will come with flash memory and then we can add a usb hard drive like the wii u.

I think this would be too cheap for a Sony console. They should stick to an internal HDD as an option, not just a usb one.