While it's nice to be able to have a "complete" language, a language that receives no updates or changes is a dead language. When people use the language, are interested in making it "better" (in their mind at least), they are building community, holding discussions and overall generating interest and keeping the language alive and useful.
If you don't allow people to make any changes, then these people will lose interest and abandon the project, and slowly over time it will die.
There are some exceptions, like C that are so embedded in everything that it doesn't matter (it still gets updates sometimes), but languages like python or java or ruby where there is no clear "best" have to be competing with one another for people to choose that language