I don't know which particular source you assume I'm discussing here.
I've read multitude of "research" (and I mark this word, as a lot of it was obviously driven by Cold War's shadows) trying to prove it, Conquest being main apologist and almost prophet of this school of thinking. It all muddies up a little by the fact that 1912-1913
were very good for the Russian economy, so casual Cold Warrior goes like this "if only this development continued until 1940, Imperial Russia would be more developed". Yeah, right. Those guys had never heard of Depression or have very selective memory... But generally they don't fit my very personal definition of impartical research.
Ahahaha, funny... I did not say Magnitogorsk was "useless". That was you putting words in my mouth. I was thinking of generally vast projects, since you were claiming that "Stalin was cool towards 'prestige projects'."
I guess we have to agree about definitions first. For me "prestige project" is something with no material ROI (although propaganda effect should be taken into account), money spent on "bells and whistles". For example, buying Mercedes as opposing to American luxobarge. Same amount of car for twice or trice the money. Gigantic projects is something wastly different. It is "invest big, gain much" type of stuff. Using car buying analogy, it is buying passenger coach instead of minivan to operate busy route. You pay more, you get more. Therefore Palace of Soviets counts as 'prestige project' in my books, Magnitogorsk does not.
What were whole new cities in the middle of nowhere if not 'prestige' projects?
Industry centers to process vast mineral deposits close to mining site, as opposed to railroading ore thousands of miles? Kiruna rings a bell?
Yes, Magnitogorsk was useful, but of the massive projects I was thinking of, that seems to have been one of the few exceptions, not the rule.
I would be curious to see other examples. Otherwise we have nothing to discuss.
Appelbaum, blah blah blah...
Obviously, that is not necessarily conclusive. It doesn't paint a picture of the canal as particularly useful, though.
If Appelbaum is reliable source for your Russian coverage (and especially for Soviet history), I would suggest you subscribe to Al Aksa TV (owned and operated by Hamas) for reliable and unbiased coverage of Israeli life and politics

Seriously, they both are equally cool-headed and unbiased. Case in point is her writings your posted here. Especially if you aware that in 1985 (when planned economy, which was environment canal meant to operate in) was at peak of it's capacity, 8 mln tonnes of goods had been shipped through it. To put that into perspective, Dortmund, being busiest freshwater port in Europe, processed 6.2 mln tonnes of stuff at it's peak capacity. BTW, canal also gave Soviet navy ability to move it's most useful ships (destroyers. subs, various cutters etc.) between Baltic and North too. So, even if it was less useful for the USSR than oil deposits are for Saudi Arabia, I would not think it is fair to call it "economically useless project".