There's nothing that inherently ties indexing and cap weighting together except that cap weighting is just the most popular.
Market cap weighting has lower turnover. When a stock doubles and another falls, market cap weighting does nothing. But equal weight ETFs like RSP rebalance quarterly, so they need to sell half the stock that doubled, and buy more of the stocks that fell. Note that within a quarter, the weights actually drift according to price moves - just like cap weighted funds. If you think equal weight is merely a preference, you can look into why equal weight funds don't rebalance more often.
I think there's merit in the idea that the rise in popularity of index investing may be increasing systematic risk for megacap stock prices. It's a cycle where the indexers buy more megacap stock because of the weighting, which in turn increases the cap, which increases the weighting, which causes indexers to buy more, and so on. If that cycle were to be interrupted, the price inflation could unwind dramatically. So I moved to equal weight to mitigate that risk in my large cap allocation.
Market cap consists of price times shares. Prices change all day long, while share counts are stable for months - so it's mostly prices driving changes in market cap. I dispute your claim that passive indexers drive the price of megacaps higher.
First, they buy equal percentages of all companies. New purchases might be 0.01% of Apple and 0.01% of the smallest company in the index. Passive indexing, by definition, is proportional. It can't target megacap exclusively - that's not indexing.
Second, passive investors get involved rarely, while market makers and HFT firms are involved in the market all day long. If passive investors all bought on one day, and a week later earnings come out, the company stock will reflect earnings - not what happened a week before. So even if indexing wasn't proportional, it's a very small amount of price discovery in the market, and less significant than it looks.
So I disagree that passive investing is driving megacap prices higher while leaving other stocks unchanged.