Once LTC gets segwit, all LN development will move there because who wants to develop under crippled mode (no segwit) when you have LTC, which is a BTC clone which actually scales and has the network effect of the original alt?
There is only one problem with your scenario - hardly anyone uses Litcoin. In the last 24 hours were only 6,798 litcoin transactions. see
https://bitinfocharts.com/Why on earth would all the LN development move to a coin nobody uses, and hence no fee profit can be made for it
because nobody uses it?
As things stand only ETh is a possible threat to BTc, because people are using it (there were about 87,000 transactions in the last 24 hours on ETH)
ETH advantage over BTC is gone now, I read somewhere that BTC will have its own smartcontract. Aside from that ETH had been manipulated ever since.
Litecoin will be Bitcoin's testing ground but with this I think it really will benefit litecoin because of the exposure and attention it will get.