What do you mean by 'decadent' here?
All Westernized peoples are incapable - despite all the ressources available - to replace themselves.
If a plant does not grow despite all external needs are met then we say it is sick, wouldn't you agree?
This sickness exists only for the last couple of generations, so it's clearly not genetic and therefore cultural, i.e. decadence.
Why should the 'decadence' of a language stop you from using it
Because it corrupts the children, plain and simple. It is the difference between your children and especially the daughters "seeking careers" (i.e. the decline or extinction of your family-line) and sleeping around (i.e. using unhealthy anti-baby pills, abortions, risk of sexual diseases) or starting families (i.e. the growth and survival of your family).
Just look at the USA: The founding population will become a minority in about 20 years. Are the people concerned about that? No, they even cheer on their own destruction.
In the 1960s, the murder-rate tripled and rapes and other crimes increased similarily. Are Americans concerned about that? No, they think the 1960s was progress and good. They don't care about their women being raped and murdered. In fact they even hide the identities of the perpetrators and concentrate on fake rape hoaxes instead.
The US constitution starts with "We, the people [..] and our posterity", but Americans don't care about their posterity anymore and the US has been turned into an "idea-nation", i.e. no nation at all.
Peoples willing to survive will only be able to do so when they distance themselves from Western mainstream culture and a different language is one way of doing that. There are other ways (religion, ideology, etc.) but a different language makes it much easier.
if it helps you pass your message across clearly, and if it does so better than many other languages because of its rich vocabulary?
The question is, is that advantage really worth all the above problems? When your daughter is depressed because of the anti-baby-pill and is becoming a drug addict because of it (I'm of course talking about 100% legal drugs from your doctor) is a job with a little bit more salary really worth that? There are millions of women in the US who are chronically depressed and are on drugs. And even if they can hold a good job for quite some time, may their drug addiction, their mental instability and their general unhappiness also endanger the "good job" they hold?
Millions of women take hormones to make them sterile and other drugs to combat the side effects of these hormones. And people don't question that, they say it's normal, that it's "liberating".
If you would do something like that to farm animals you would get jailed for abuse.