To be clear, I personally use branching a lot (e.g., for sprint-sized features) and I create new feature flags rather infrequently (for multi-sprint sized features or for A/B tests), and only for pretty large features. But I can see reasons why some teams/environments do go, or want to go, feature-flag always. Before git, feature toggles were more widespread as a workflow.

As far an entanglement with other code, I would evaluate this wrt branching. If it's entangled, the branching analogy is a merge conflict. The most common case is you can delete the toggle without conflict, and occasionally you have to spend ten or twenty minutes untangling. Every once in a rare while it gets messier.

For testing, the prod tests run the prod config, which is likely with all optional feature toggles turned off. My team would run our own tests with our feature toggles turned on, but not with other teams' feature toggles. I don't run all combinations of features. Usually, a feature toggle will also wrap new tests that belong to that feature.

Again, the same question you ask applies to a branch workflow. Do you run all possible branches at the same time? Is it an exponential combinatorics problem to have multiple branches? Not really, each team runs the tests for the features they work on. Same either way.

The great thing about feature flags for testing is that you have greater control over who is exposed to your new feature. With a feature toggle, you can expose some teams in your company outside of your own, without exposing the whole company. You can expose 10% of your customers and do testing without inflicting bugs you didn't forsee on 100% of your customers. If there's an emergency, you can limit the damage radius with a well designed feature flag. Branches can do this only to the extent that the merge structure allows it, and they can't help with customer testing.

> It is so far my suspicion that most people who speak of feature flags as an alternative to branches either work with trivial products [...] or talk out of some theoretical conviction which has yet to meet with reality

I think I know why you feel that way, but that sounds (to me) like you haven't been exposed to (and so can't imagine) workflows outside of git branching. I mean no insult, but you are jumping to conclusions from lack of experience. Feature toggles as a pseudo-branching workflow predate git by decades, they are used on the largest codebases in the world -- did you miss the comments in this thread from MS and Google?

Feature flags are used in nearly all repos where branching exists. It's not one or the other, you can use both workflows at the same time. A/B testing is based on feature toggles, and all sizable websites on the web are doing A/B testing constantly. Chances are high that you use them already and just haven't yet recognized that they're the same thing we're talking about. If you have config files for your project, then you have feature toggles. If you ever change a value in one of them, then you could instead branch your code, remove the config variable and rewrite the code that used to reference the feature flag to be hard coded instead. Is that making sense?

FWIW, just to put some specifics and back up my own 'conviction' with real world examples, I've been on teams that used feature toggles to test and ship features during the production of several animated films, movies you probably saw. When I worked in film, it predated git. We had a hierarchical repo, but there were no branches, only pushing to master and feature flags.

When I worked in games, the team I lead used feature flags to support cross-team testing on a $1B console franchise.

At a certain web company, in addition to constant A/B testing, my team shipped features to customers who signed up to be on the 'beta' versions of features before they were rolled out to all of the >1M users. We used feature flags to control which teams internally could see & test certain features, and we used feature flags for either long-running or large cross-team features when branching & merging would cause undue amount of branch noise.

And at my own startup now, I prefer to wrap my features in feature flags even when I'm branching. That way I can back out a feature when it causes problems or has bugs without having to do git surgery or even risk a merge conflict. With a few thousand active customers, this has saved my ass multiple times.