Starcraft 1 mapping and modding will never flat die. It is easy to use, virtually free, and works on older computers. For people still using old computers, it will still be a preferred game. That being said, the audience has shrunk, and continues to shrink. At this point, a meaningful chunk of the SC1 audience is people who are on "1-gaming system" set-ups, and with an old computer that means that when their old computer dies (as it will) they might not ever come back. At which point they will eventually get a new computer, and with it, probably new games.
Since pirating of games has gotten so extreme, as the status quo for machines and video cards moves further into the next generation, people will be less tempted to come back to "old/free" games that they already own and will instead make the leap to pirating newish games.
As such, the Starcraft 1 map/mod community will continue the trends it is already experiencing:
-some maps made by people who have been mapping for over a decade will continue to be released: high quality, but with low visibility and a low audience this demographic will continue to move to newer games for their modding projects (dragon age: origins, sins of a solar empire, etc.)
-some new players who have inherited or bought old, cheap computers will be attracted to it as a game they can play for free. But, since they are so far behind the 'curve' in mapping technique, they will be ignored and thus have a low purchase on the franchise
-a few maps might come out on SC1 as "demo maps" for SC2, as a way of getting core gameplay mechanics down or to build publicity for major projects... but these will be few and far between, and on the whole map and mod production will continue the nosedive it has been in since 2005.
-The mapping/modding community will nevertheless serve as the established space for Starcraft 2 Modding/Mapping to find its foothold, but all the attention will be on SC2.
-There will also be nostalgia value as most of the "hits" from SC1 will be reproduced in SC2
Personally, I think that not only the "classic" SC1 maps and mods should be preserved, but also the later-era (post 2005) maps that showcase the new developments in modding and mapping leading up to SC2 should also be highlighted. Whether or not the "community" for SC1 mapping will continue seems a moot point--it will, under the new banner of SC2, and the SC1 community is already basically dead.
I will accept the transition to Starcraft II, as soon as I get a computer with a graphics card that can actually play it. And yes, the change will be permanent, for the good or the bad under blizzard's new content sharing policies.
None.