Water infrastructure is old, critical and already quite fragile, I simply don't see the upside here and I see a lot of ways in which this could go wrong and/or could lead to future trouble. It is - in my opinion - an overly complicated solution which risks something that is far more important for marginal gains. If a deck proposing this would be presented to me as an investment thesis I would suspect the 'success' to be the investment, not the long term deployment of a working system.

Being pessimistic about bad investments isn't counter productive from the point of an investor, it is a net gain because then you get to spend your money better.

"Wouldn't it be nice if we had 'x'" is rarely a good reason to invest in something, see 'Theranos', 'UBeam', the 'Moller Skycar' and a ton of other things that it would be nice to have but that are not feasible in practice.

> "Even if it turns out that only 10% of the water infrastructure in the UK is usable for this sort of expansion, it has the potential to be a useful technique."

Where does that 10% come from? All I see in the article is that they are trialing this and no mention of any percentage of the water infrastructure that might be suitable for this is made. See my other comment for one way in which you might get this to work which avoids some of the downsides but it still won't be cheap or easy to maintain long term, and it will always require coordination between the utility and the fibre company in case of a water mains issue or a fibre issue.

Edit: I've found an old article detailing a trial in Bonn, Germany, I wonder what came of that:

https://technical-news.net/bonn-fiber-optic-cable-laid-throu...

So this is definitely not a 'first' as many articles about this project say.

Bonn, like the i3 and other mid/late-2000s UK sewer infrastructure installations, failed to even meet the baseline metrics set by first generation micro trenching fibre rollouts - which, as we know from Google Fiber in Louisville, KY, is an extremely low bar to hit. Micro trenching was a decidedly inferior solution to instead utilizing existing above-ground conduit or doing proper in-ground installations with full fat trenching or similar methods. Google’s micro trenches failed so badly in Louisville that they fled the town and paid $3m+ in remediation fees; this is the tech that is SUPERIOR to sewer installs, according to the results from Bonn and other Western European municipalities.

TL;DR: AFAIK, there is not a single trial run that has found sewer to be the best option for any of the following metrics: initial installation cost, ease of installation, lifetime cost/TCO, maintenance frequency, and network downtime/reliability rates.

That is totally in line with what I would expect, I see these companies as just milking the trial money from gullible mid level governments.