Surely there's more to it than 'well it seems people can understand each other, -> effectiveness = 100%'

That's really it. Every language is equally adapted to meet the needs of its speakers. For an example, the Pirahã people of South America used to not consider numbers to be relevant to their culture, and thus they didn't have a number system in their language. Having numbers in their language would have been redundant, as they would rarely use them. However, once they began sending their children to school, their children borrowed the Portuguese numbers because they became relevant to what they wanted to talk about.

'The speakers' are not a homogenous group though

Neither is the language. You can speak however you want, other people can speak however they want. Languages aren't monoliths; they're nothing more than collections of somewhat similar speech patterns. Whatever you seem to think I'm saying about "were" isn't true; I'm completely fine with the usage of "were" for the subjunctive. I'm also fine with "was". What I'm objecting to is calling one incorrect and the other correct.

That is simply logically not true. Every time I use 'were' as a subjunctive, and someone understands what I mean, the word is not dead yet. If enough people do this, it keeps on living.

People still routinely use "thee" and "thou" (or variations thereof) in some places. But singular "you" is considered correct as well. Similarly with the subjunctive, just because you happen to use a more conservative construction doesn't make everyone else wrong.

You cant debunk that wholesale though, it will vary on individual cases, it even has a subjective element to it; you cant debunk that I value having distinct words for subjunctives.

I'm talking about the inherent value of constructions in language, not biased opinions.

Except that it requires more cognitive effort to identify a subjunctive. Minute as it may be, thats a loss in my book.

Then I suggest abandoning English and choosing a far superior artificial language such as Lojban or Ithkuil to ensure you never have to hear another ambiguous sentence again.

In all seriousness, though, maybe it does for you, I wouldn't know. But that's because "were" is part of your idiolect. On the other hand, people who use "was" in both cases don't perceive a difference between the two moods. It takes no more effort if that's how your idiolect treats it.