Every mile of track is sacred...!
I agree if a single section has been sufficient for decades but needs redoubling now then even with the benefit of hindsight it was not a bad decision. Some single sections like Dore are a major problem and redoubling is necessary. The Windermere branch is a good example of a singling that was probably a good idea. Id say it needs longer platforms and either electrification or battery / hydrogen trial much more than redoubling.
Windermere is fine - it's an example of a singling that was done in support of a specific service pattern (hourly) which is correct for the line's level of demand and it does allow that service pattern to be delivered punctually and reliably. The only point of redoubling would be if you wanted to allow for charters on the line, which is not a justifiable reason for public subsidy of such changes. Realistically if the branch gets busier the best solution is longer trains on the existing service pattern.
Similarly, Ormskirk-Preston was singled with a passing loop at Rufford, supporting the two-unit hourly service operating at the time (though further cost-cutting meant that the service was dropped to a single unit irregular one, sadly). The Island Line also ended up with the same problem - two loops were built supporting a three-unit, 20 minute frequency service which proved to be overkill. But as long as you specify the infrastructure for the service that will be needed long-term and don't fear doing the necessary work to change it if that changes, the concept works. Lausanne, for example, has a tram line which is mostly single-track! Also most of the Swiss narrow gauge lines are single with loops at stations.
St Albans was however a piece of idiocy - infrastructure inadequate for a half-hourly service, but hourly is seen as inadequate, so you get the bizarre 45 minute frequency instead, when half-hourly (2 units) peak and hourly off-peak would make most sense.