I don't think .38 or .357mag are going anywhere, they represent tradition (generally important to revolver owners and shooters), and work well in revolvers. Though, I've been playing with the idea of going with a 9mm snub for years.
In a K-frame revolver with a 4" barrel, .38spl hits above its weight class IMO. Despite somewhat mediocre power ratings (ft lbs), it has a good record with the right ammo. In a 3" or 4" barrel it penetrates well, without overpenetration, and a decent hollow point will reliably expand at the velocities generated in a 4" barrel and will usually expand in a 3" barrel. I love 9mm, but it really depends on the hollow point to expand for its effectiveness. When clogged with winter clothing, not all JHPs will expand. If an old tech LSWC-HP doesn't expand, the sharp shoulders of the SWC will work better to stop the threat than an unexpanded JHP acting like a FMJ. With its lower pressure sound wave, you also have less potential for hearing damage if you have to shoot it indoors. IMO, the .38spl is one of the best cartridges for home defense for this reason.
In a 3" to 5" revolver, there may be no better all around outdoorsman round than the .357mag. Enough power to stop most North American predators (though, I'd want more in grizzly country, and against the larger grizzlies and pretty much any polar bear it may be about useless), with far less recoil than .44mag or above. You can get far quicker follow up shots than a more powerful magnum, and .357mag is quite manageable in an L-frame or larger revolver, and quite acceptable in even a K-frame. Meanwhile, in 125gr JHP or SJHP form, it has historically been one of the most effective self-defense cartridges. A 3" to 5" K-frame or L-frame isn't too hard to conceal OWB, especially open carried, and the 3" K-frame (and for some the 4") is reasonably concealable.
In a carry snub however, a 9mm may well be a good answer. In a 2" revolver, it can be tough to find a round that reliably expands in .38spl. There are a few, but not really all that many and that can become a problem when there is an ammo shortage. Traditionally, the answer has been WCs or SWCs, but then you lose any chance of expansion (I like LSWC-HP to have some chance of expansion while covering my bets if I don't get expansion). In .357mag, you get to some pretty severe recoil levels in a small framed snub which can cause quick follow up shots to be difficult. Modern 9mm loads expand pretty reliably in short barreled handguns, and will about cut the difference between .38 and .357mag in recoil, while getting closer to .357mag power levels out of a 2" revolver. Moonclips provide much faster reloads than speed strips or speed loaders, and 9mm revolvers don't need you to spend extra to have them converted to moonclips.
There are three disadvantages though. One, there aren't many options if you want a revolver in 9mm. Also, they are generally limited to 5 rounds (I have a 6 shot Taurus 856UL that is roughly the same width as the Taurus 905). Last, you can only get a steel framed snub in 9mm, if you want to go lightweight, you have to look elsewhere (Micro-9 pistols, .38spl aluminum revolvers, .357mag scandium revolvers).
For me, I've thought about it for years, and I'm starting to lean towards picking one up in the near future. I do like my .38s, but I have more confidence in 9mm. I like roughly splitting the difference in recoil between .38spl and .357mag with only a slight penalty in power (when compared to .357mag in a 2" barrel). Over the last couple years I've started to really like snubs and 9mm in a steel framed snub shouldn't be too hard for me to manage quick follow up shots. In autos, I mostly shoot 9mm (I do have, and love, guns in .45 and .40, but the only caliber I buy in fairly large quantities is 9mm). There have been times that I've wanted to bring guns in other calibers to the range and found I shot up the caliber it came in and didn't replace it yet (including several times for my .38spls and .357mags), I never run out of 9mm. That is another good reason to go with a 9mm snub generally (if you already have a lot of 9mm for your autos), and I am definitely in that camp. If I do it, it will replace my steel framed 5 shot Taurus 85, not my 6 shot and lightweight Taurus 856UL, 6 shot Colt King Cobra (new model), or lightweight and very pocketable S&W 442.
