Who can really predict what the future holds.
Ethernet has been changing so I wouldn't say it is the oldest. Early ethernet was ran over coax cable.
BluRay just barely beat out HD. Who knows what will be next and if it will be backward compatible.
Also, most computers are not coming with optical drives anymore. Heck, a lot of laptops are not even coming with wired Ethernet.
My concern with BluRay, specially 100 GB, is the time it will take to write to that disc. 100 GB from SATA to SATA hard drive takes about an hour. I would guess that to BluRay it would be more like 3 hours.
For those who used to like Retrospect, there is some good news ahead, a few of the original programmers took Retrospect back over from EMC2 and have started to improve it again. EMC2 basically had no incentive to keep Retrospect up and running as they wanted to push people over to their enterprise stuff. Retrospect saw a few updates to keep the install base happy, but it fell behind in many ways. It *should* get better again.