To my surprise, my fears were misplaced. Go-Flex lives on but in a different name and different format. It is thinner, smaller and more compact. It is called USM (Universal Storage Module). In fact, there is a newer Thunderbolt Adapter (STAE128 instead of the STAE121). Seagate didn't do a good job of informing the consumer about this.
Well today, I picked up a USM drive and I didn't even know about it. The Go-Flex drives have been replaced by the new line of Backup Plus drives. They're thinner and more compact as seen below.
Compared to an original Go-Flex drive.
Here it is attached with older Go-Flex attachments like the Thunderbolt STAE121 adapter.
Old Go-Flex Firewire adapter works just as fine.
As I wrote earlier, Seagate didn't do a good job of informing consumers about USM. After a few googling, a few other companies are implementing the USM format. Here are some examples of other USM products in the market place. There are Pogo plugged devices, desktop computers with slide-able docking bays, media players and NAS servers that now employ USM.
(image from Anandtech)
USM is basically a small format enclosure for 2.5" drives. They have interchangeable dock connections via standard SATA. The key thing is the standardized size.
When you look for an enclosure or portable drive, give USM a consideration. The ability to switch between USB 3.0, Thunderbolt, direct SATA and future connectors is pretty compelling.
Some reading links on USM: