Let's look at a numerical example:
Assume Tesla is 10% of an index.
Now, Tesla falls 50% and all other components of the index do not move.
Index decreases to: 90*1+10*0,5=95
Tesla share of index is now: 5/95=5,26%
Let's say Tesla was worth 200 $ and the index 2000 $. (Tesla 10% weight)
The index owned 1 Tesla share for 200 $ and 1800 $ in other shares.
After the price decrease, the index owns 1 Tesla share for 100 $ and 1800 $ in other shares. The index value is 1900 $ now. (Tesla 5,26% weight)
Hence, there is no need to rebalance based on the price decrease as long as the index owned the appropriate amount of Tesla shares in the first place.
As mentioned in an earlier
post, Elon's sales do increase the free float, which ultimately results in index funds buying more Tesla shares (However, I am not sure about the timing)