I dunno...what does the RUM do that the 30-378 didn't already do?
I think the Dakota did/does fill a niche. It gives you 300 Weatherby performance in a standard/30-06 length action, and no belt. It gives you a 100-200 fps edge over a 300 Mag in a slightly shorter case. How isn't that an improvement or niche, like you mention? They made necking down the 404 Jeffrey cool, long before the RUM came along. They did just what you mentioned others did - they gave top-of-the-line performance for a given length action (better than 300 Mag), and matched the performance of some rounds which required a longer action (300 Weatherby).
Also, for bullets up to about 180 grains, they are very close in velocity to the RUM with 10-20 grains less powder. I had a very knowledgeable gunsmith who pointed all this out to me when doing a custom build for me a dozen years ago, and I ended up picking the Dakota for that reason. Why shoot 95 grains to get the same velocity 80 would give me in the Dakota? Except for the heavy, longer-range bullets, the Dakota really does hit a sweet spot of performance - to get the next 100 fps takes about 15-20 gr more powder with a 180 grain bullet and 26" or so tube. Now, if you shoot 220 gr+ and put 28"+ tubes, that changes, of course. However, for the average (not long-range) hunter - you are at near-RUM performance with 180 gr bullets and below, in a shorter, lighter action, and you have a lot less muzzle blast. What's not to like?
Now, if you'll look on another recent thread, you'll see I'm not a Dakota lover, in fact, I steered the guy to the Norma. This is the long-range forum, and if you are wanting to shoot the heavier bullets, the RUM certainly does pull away in performance vs. the Dakota. As I also pointed out, Dakota brass isn't great (up and down, depending on the mfg-of-the-day) and it's very pricey. For those two reasons, and the fact that since building it 10+ years ago I've figured out that sometimes the nice bulls stand at 900 yds vs. 600 yds. - I probably wouldn't choose it again. If I could get Lapua or even Norma brass for it (Norma did make some for 7mm and 338 for a while I think - but no more) - I'd like it more. If I was a "normal" 600 yd and less hunter and they had good brass - I'd almost definitely choose it over the RUM .
In any case, I'm not in love with the girl any more, but I still don't like to hear her falsely accused or slandered. (I did a LOT of homework on it - I didn't just take the smiths word.) I think it does fill a niche, or at least initially did, and I think if they had good brass it would be an excellent cartridge for 700 yds and less, and the 7mm Dakota would be awesome for deer. However, now...you take a round that isn't the fastest, isn't factory chambered, and doesn't have good brass - no matter how good or well-balanced for caliber...the outlook doesn't look rosy.
