> reaching a peak by the 1860s and declined from then onwards with the increase in steam power and increased mechanism.

Lloyd held a similar view, which seems plausible on the surface. (A decline in tasks requiring the use of shanties results in the decline of shantying.) But I'm not sure what "decline" means here.

Were fewer shanties introduced after 1870?

Were there fewer shanty "performances" after 1870? (There were more machines, but there were also far more ships and sailors!)

Were earlier shanties "better" (more musical, more coherent, better "written")?

Did the lyrics of shanties begin notably to fragment and decompose? Did the "standard versions" get shorter?

Were post-1870 shantymen less proficient? Were their repertoires smaller?

Etc.

It seems impossible to answer any of these questions. The necessary research was never done, and probably could not have been. There isn't even much basis for conjecture.

How many of Hugill's extended lyrics were fleshed out by his own creativity (presumably mostly at sea)? We don't know, but the brevity of most other collectors' texts certainly raises questions. At any rate, there must have been a fair number of creative shantymen whose personal innovations, clean or otherwise, never traveled very far.

It's interesting to consider Rutzebeck's rather literary texts in this light.

All we can say safely is the obvious: that by the time big sailing ships became few in number, there was far less shantying being done.