[comments interspersed between original text bits]
 
Yet IMO conlanging is going on at all times in the world
of natural language. People coin new slang terms constantly,
and new inventions like "radar" and "modems" produce new
words than may become quite common. All this may not
be serious stuff. Some is clearly for fun -- take "Jabberwocky"
-- not without its influence on the English language.
 
[I'm not sure that I would call adding vocabulary to an existing language conlanging, but that's just me.   I suppose one could speak of neologisms as a part of constructing the language of the future.]
 
Natural languages are "under construction" constantly.
Moreover, the leading "natlangs" are all to some extent the
result of conscious efforts to define norms and establish
a common vocabulary. Famous poets have set the
standards for many -- including Pushkin's role in forming
modern Russian; Dante in Italy, etc. Hungarian vocabulary
was in part an artificial creation of its pioneer lexographers.
 
[New vocabulary comes from a lot of places;   neologisms are only one source.   A lot of roots are traceable to ancestoral languages, even if we disregard reconstructed proto-languages as mere explanatory constructs.   Then there's loanwords, etc.]
 
Hebrew came back from the dead -- reorganized by
planners. Norwegian was restructured (from Danish)
and reorganized (at least twice). Most literate African
"languages" today were created by conscious choice
by language planners by selecting from leading tribal
dialects, often fusing them into a norm that is then taught
in the schools. Missionaries did the same with Fijian
and other South Sea languages.
 
[These are impressive examples of language planning, but all of the planned languages in question are based on languages whose creation and development were not planned.]
 
An Emperor of Korea invented Hangul; Kemel Ataturk
reorganized Turkish, replacing much grammar and
vocabulary and putting it all into a new alphabet.
 
[Writing is unquestionably an invention.   It isn't found in every human society;   at one time it was found in vanishingly few.   Spoken language, on the other hand, is found in every human society other than the deaf culture.   This includes illiterate societies without language planning, of which there have been many.]
 
To imply that languages "just grow" is IMO very
nearsighted. Even the major "school grammars"
show the impact of the conscious efforts of
individuals on the direction of language.
 
[Unquestionably, languages in illiterate societies develop without planning.   In literate societies, language planning can define the dialect used in formal discourse, and even constitute a source of vocabulary.   But to say that these constitute the invention of languages such as English, French, German, etc. is to overstate your case.]
 
"Classical Latin" "Classical Literary Chinese," Sanskrit,
etc. were all norms set by "grammarians" -- not mere
refections of what was being spontaniously spoken by
some population. Further back, languages like Akkadian
seem to have been "devised" from various souces by a
series of planners.
 
[Typically, the norms come from great works of literature, although the authors of such literature are occasionally those who codify the norms, as with Martin Luthor and Dante.   Again, however, specifying an educated or artistic dialect is not the same as inventing a language from first principles a la Lojban;   educated dialects are based on ones that "just grew."   As for the idea that Akkadian was artificial from the phonemes up, that's a fringe claim AFAIK.]  
 
ASL and all the other sign languages for the deaf are
clearly inventions. So were things like Gregg shorthand.
Also things like Bliss Symbolics.
 
[Pidgins are also inventions!   ASL is a language.   Gregg shorthand isn't;   it's a way of writing a natural language.   Bliss Symbolics?   More iconic than natlangs, and not as flexible as natlangs & creoles.]
 
Natural languages and subsidiary language forms are,
in fact, often the result of conscious efforts to fill a need.
And fun can be a need as well.
 
[Well, inasmuch as pidgins and invented sign languages have happened often, I suppose you are right up to a point.    But most natlangs are not inventions.   Inventing a lingua franca can be like taloring;   a language can be made from whole cloth.  But most of the language-planning successes you've described are more like hair styling;   the stylists shape the hair, but did not invent hair.   Ben Yehuda altered Hebrew, but did not invent Hebrew.]
 
[Most imporantly, language planning does not have a one-way influence on natural language.   Pidgins, for example, become creoles, incorporating many unplanned changes as the use of the pidgin "just grows."   Standard usage, in English for example, also changes in accordance with unplanned changes in popular speech.   We may both live to see the day when "whom" becomes unnecessary in formal discourse, precisely because it's dropping out of the vernacular.   We may even see further unplanned atrophy in the English subjunctive reflected in educated prose.]  
 
[Finally, some grammatical norms are just B.S.    Nobody accepts the double negative in formal discourse, but no one interprets it as affirmative either;   the "two negatives make a positive" dictum is
simply untrue both in non-standard English and in standard Russian.   We've all heard that sentences aren't supposed to end in prepositions;  but this is a restriction "up with which I will not put" in the words attributed to Churchill.   Happily, not all scholars reject the use of "hopefully" as a sentence adverb.   Sadly, some grammar pundits still don't know what a sentence adverb is, but hopefully they will learn.    Also, while infinitives are often impossible to split in classical languages like Latin, there is no good reason by William Shatner can't say "to boldly go" in English.]  
 
[It's important to distinguish natural language from formal discourse.   Formal discourse is the use of language that produces literature and technical or artistic communication.   Language, we acquire without instruction;   formal discourse has to be taught.   However, the former presupposes the latter, even as playing in a symphony orchestra presupposes basic manual dexterity.]
 
Jim