Does that mean that Bitcoin + LN will kill many altcoins such as Bcash, Litecoin? What will be the altcoins' next arguments to dominate their main competitor - Bitcoin?
It's always easy to spot a Blockstream shill by the way they refer to Bitcoin Cash in the pejorative.
So, to answer your question - YES. As soon as the lightning network becomes part of bitcoin, I think there won't be a need for alt coins anymore. Unless it's something totally unrelated to digital money.
Also less energy consumption and ease of use are big points. Many alt coins will eventually fail I think, but at this moment they seem mostly linked to how well Bitcoin is doing, so if Bitcoin goes up or down, so will many alt coins at first.
A more likely scenario is that Bitcoin remains a store of value and people exchange Bitcoin for some other cryptocurrency like Litecoin which they'll use for their everyday purchases.
Yes, coins like these will need to innovate somehow or their utility will drastically decrease.