There are lot of crypto currencies that were created to compete with BTC
BTC lightning network is implementing due to the scalability issues the chain is experiencing with high 'fees' and transfer times..
If lightning network is implemented in Bitcoin will it destroys these competitors?
If just one new feature of Bitcoin, like Lightning network, would make certain altcoins disappear, then those altcoins were nothing but useless clones of Bitcoin. However, some of those "devs", unless they bought pre-fabricated clones, may also add LN capabilities.
But in most cases LN deployment in Bitcoin network will have no effect on the altcoins. Just think: would Bitcoin LN make Monero less anonymous, or would make smart contracts in Ethereum, Waves, and so on networks work less good? Or would make HDD mining / storage less effective? No.
So, shortly, the real altcoins don't care what happens on Bitcoin network; the only one that could be affected would be the worse clone shitcoins.