Why languages don't necessarily cannibalize / destroy others

Another discussion on Auxlang that I just responded to that I think is worth making a post of as well. There seems to be a certain assumption that languages simply cannibalize others, where large languages move in, destroy smaller languages, become more influential and then end up moving on to larger and larger languages until eventually the world has a single language. I don't see this happening. Here's part of the thread, with my most recent response at the bottom:

>> Yes, in the future that might well happen, but what I really
>> meant, and I think you agree here, is that we should be
>> planning for L2 speakers. If people start to speak it
>> natively, then so be it, and the language will probably
>> change as a result.
>
> The language will start to change as soon as people start
> speaking it. Once it becomes a living entity the evolutionary
> process will kick in.
>
> Any auxlang that becomes popular will have a native population.
> Just as there are a few "native" Esperanto speakers today,
> you'll eventually have people with spouses that speak a
> different language so they'll resort to using the auxlang at
> home. The result will be children who speak it natively.
> Eventually, this will lead to auxlang monoglots because the need
> for any other language will continually decrease.
>

That's not the way languages work. The idea that languages simply cannibalize others in accordance with their relative influence is only true part of the time, when conditions dictate that without language x a person simply can't make a living, and when parents don't feel the need to teach their children the language. When you have people living in relative prosperity though the opposite occurs:

The most recent census figures (2001) presented in "Main Statistics about Welsh"^[8] by the Welsh Language Board, indicate 582,400 (20.8% of the population of Wales in households or communal establishments) were able to speak Welsh and 457,946 (16.3%) can speak, read and write it. This compares with 508,100 (18.7%) for 1991. Increasing use of the English language had led to a decline in the numbers of Welsh speakers. Since the introduction of the Welsh Language Act 1993, giving Welsh equal status with English in the public sector in Wales, the Welsh language has enjoyed a revival.

The results of the "2004 Welsh Language Use Survey" indicates that there are 611,000 Welsh speakers in Wales (21.7% of the population living in households, a lower figure of 19.7% is given in the same paper), 62% claim to speak Welsh daily, and 88% of those fluent in the language speak it daily.^[8]

According to the cannibalistic theory of language Welsh should be getting progressively weaker, but this isn't the case. Then comes the argument that 'this is just government support and without it it wouldn't be growing', but there's always government support with any language (large ones receive the most) so to pretend that only small languages are receiving government support is also not correct.

Secondly, learning a second language makes people more predisposed towards learning third languages and beyond.

Here 20% of unilingual people are simply not interested in learning another language, which drops to 7% when the person is bilingual. So it's actually quite rare that a person learns a useful language, then sits back and thinks well, that's all the language study I'll be doing in this lifetime. That means that as the world begins to learn an IAL and unilingual people start to become bilingual, that means that the general interest in language will also increase and people will branch out into others as well.

Also, having a world IAL will never mean that it will be useless to learn another language. There will always be newspapers, blog posts, broadcasts, shows and so on in other languages. Seinfeld doesn't work outside of English, Burhan Altıntop isn't funny when not in Turkish. Let's say the world language is Interlingua and I'm a businessman on a trip in Quebec from New York. Everybody speaks Interlingua and that's great. After work though I hear some people on the street arguing in French, or a street play in French, or two kids are saying something to me in a loud voice from a distance (let's say it's 'look out for that bus behind you') because they don't know I'm not from here and I'm not sure what they're saying but it seems to be important. In none of those situations can I ask them to switch over to Interlingua. Well, I possibly could in the third but the fact remains that I've missed a very important piece of information through not knowing French. So there are a huge number of situations where knowing some of an extra language will be very helpful, and hey, Interlingua's a lot like French anyway and so I could learn some of it pretty quick...and voila, you have the opposite of what you've predicted.

Let's also not forget that if a language like Interlingua starts becoming the world language people are going to start becoming more interested in Latin again, because it's already pretty easy to read, and there again we have a dead / nearly dead language getting a jolt of adrenalin where the cannibalistic language theory assumes that it would simply be ground further into the dust.