If I buy a fixed portfolio of asset of ETF indexes for many years but some of the ETF indexes are not doing well in some years, should I sell and buy others or keep buying to rebalance?

Depends on which ETF indexes you are buying. If you believe that the index will grow in future, then you can keep it. Otherwise, I will sell it off and restart if I admit i made the mistake in the past.

If you have been dca into the index, you should continue buying into the dips as once the market recover, you will be sitting on capital gains and market tend to rise over the long run.

A lot of people recommend rebalancing to reestablish Your initial risk/asset allocation.

I disagree: Before investing elaborate your strategy and allocation, then stick with it ultra-longterm.

(maybe all you need is ticker: VOO or IVV)

If you believe in reversion to the long term mean, then yes, you should rebalanced. However if u believe there are fundamental shifts that have permanently resulted in the lower performance, then no.