The index funds don’t care so much about what price they have to buy. What they care about is what’s gonna happen after inclusion, in particular the last days of December trading. Their business depends on S&P 500 delivering a solid annual return (10-15% or better), that’s the market appeal of these funds.
So, in my opinion, this means if the index funds have any control over what happens at all, they will focus their efforts on preventing a huge sell-off after inclusion. That’s why I’m not worried about it.
The problem is, once their TSLA buying is complete, they will just sit on their shares and have no way to influence any of the stocks in the index until the next re-balance which is March 2021 AFAIK. So it is immaterial "what is best" for the index funds, they will have no power to influence the outcome -- that's why they are called passive funds.