Satoshi & Sirius made this forum & gave the control of it to Theymos for reasons and they know how it effect on Bitcoin . If bitcointalk dies then Bitcoin will dies with it or most of it .... Basically 99% of questions with answers related to Bitcoin are here on this forums , so if there is no BTC talk then simply people won't know how to start . If it goes down people won't know how to start , a lot of websites will die with it too like exchangers & Gambling websites that are doing Signature Compaigns etc ...
There is also sites like reddit that already exist where commerce takes place and that has a lot of bitcoin related discussion, including some technical discussion.
I know that , but what you just mentioned are far away from being anything like Bitcointalk forums . Beside a lot of meetups are made here on Bitcointalk "Meetups" section unless it's too official .
Personally as I said , nothing will replace bitcointalk at all and BTC may even die because of that . Reddit is #2 after btc talk.