Your misconception stems from this fact: the "best" chain is determined not by the number of blocks in the chain, but by the amount of work in the chain. Right now nodes consider the average sha256d block to contain much more work than any block from any of the other algorithms (due to magic work factors set before asics were common).
This from
https://en.wikipedia.org/wiki/Bitcoin_network "The best chain (black) consists of the longest series of transaction records from the genesis block (green) to the current block or record. Orphaned records (purple) exist outside of the best chain."suggests that what you are presenting is a major "revelation" that stands the crypto-world on its head and that therefore needs some very serious documentation supported by hard data.
That page just uses non-technical wording. Bitcoin also uses the "most work" criterion.
Work factors only apply to multi-algo coins.
Thanks for that clarification.
One last question for now and then I'll leave this to others who may indeed know more than I do.
"Work" only comes into play when determining which of two simultaneously discovered blocks is added to the blockchain, correct?
Even if the sha256d block contains more work, if block discovery is relatively equal at 20% per algo, how does that change the theoretical percentages necessary for a 51% attack on a multi-algo? Let's say each sha256d block contains more work, but it's still a relative 20% of the total (even if someone control 100% of that 20%) meaning that an incorrect chain could not be created and the attack would not be successful. Let's say that sha256d is worth 100 times more work than any of the other algos, the fact that those blocks are evenly distributed at a 1:5 ratio within the total blockchain would completely mitigate that "superiority", would it not? It's still only one of five.
The only way I see that this could be an issue is if it were possible to get huge strings of blocks discovered by only one algo in particular while the others find nothing, and that doesn't happen with DGB.