Finally, a topic I actually have some insight on... or it least, it WAS that topic....
Anyway, DVDs are reliable, but I don't trust any media any farther than under the caster wheel on my chair. I continue to keep multiple backups, on different hard drives in case one fails and in case the removable media fails. And one copy off site, in case there is a fire which consumes hard drives and removable media on site. And yes, the technology will continue to upgrade every 5 years or so. We don't need to worry about whether a DVD will last 25 years or 50 because DVDs will likely be obsolete before either of those. I heard about a disc that is on the drawing board for about 10 years out that will hold 1.7 terabytes. Can you imagine putting 2600 of your most favorite CDs on one disc? Or 360 DVDs?
And, with DVDs being so cheap - I am getting decent quality discs for around $1 each - and with burners being cheap too, it doesn't make sense not to use DVDs.
The Blue discs are a reality. They are in production now, and could be mass-marketed sooner than you might expect. They are 27 Gb +/- depending on how you measure. I have not seen the specs on these, but I expect they will burn significantly faster than DVDs, at least in terms of MB/s. Higher density data has always had a comparable increase in speed to accompany it. If it takes 4 hours to burn one of these new discs, few people will buy them.
History shows that the market does not support double sided discs because they require a caddy, of some form of enclosure that keeps the minmum unit cost too high. Double (and possibly triple or quad layer) discs are completely feasible and supportable by the market.
Another thing the market doesn't support is more than two types of removable storage media at one time... at least not for very long. When the next level of DVD becomes widespread, CDs will begin to phase out. Of course, CDs will be the exception because they are not just a computer media, like all the rest of the historical examples for computer media. There are enough standalone CD players/mp3 players that the demand for CDs will be around for a long time, but they will become less and less popular, and less capable of handling the increasing file sizes, to the point that using them for computer media storage will be almost foolish. DVDs will be resistant to phasing out as well, but it will happen.
Kenneth