But the thing I find fascinating is that somehow it was deemed feasible to create the tunnels on the Faeroes, which have a population of about 50,000. Presumably a very different method is used by the Danish government for determining whether to go ahead with such projects.
I think it broadly comes down to an acceptance since around the 1960s that such tunnels provide a wider societal and economic benefit that isn't as easily quantified, or is worth subsidising (with a large subsidy from Copenhagen to boot - something the Channel Islands might aspire to!
) as well as reliability unlike the ferry services they replace... a tunnel can't be cancelled in the winter! It keeps people on the islands if life is a bit easier, rather than everyone leaving the villages for Tórshavn or Denmark proper.
The tolls for undersea tunnels can be expensive - £20 in one case (although there are subscriptions for locals that make it cheaper)
With regard to the two newest subsea tunnels in the Faroes (Eysturoy & Sandoy tunnels) the more heavily used one (Eysturoy tunnel) is expected to subsidise the other (Sandoy - which was always accepted as being economically unfeasible on its own) so that must help make the cost more palatable as well.
The schemes are also different in aim - Tórshavn has always been the distinct centre of an archipelago (around a quarter of the population lives there - 3x bigger than the next town but leaving three-quarters of the lot searching for how to get there!), whereas a "Chunnel lite" would connect two distinct islands as well as the French mainland. Perhaps this would make it less worth it, or more so? I'm not a professional tunnel-builder!
There is also simply a lot of experience building tunnels (cheaply) in the Faroes! The two tunnels I mention cost just over £300 million to build - per resident around £5,800. I suspect this Channel Islands tunnel would work out significantly more expensive per head, and then you've only got one tunnel!