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?
In short, no.
The good alts - by which I mean those with a solid real-world use-case, a good team and a clear vision - offer plenty of their own, which bitcoin, with or without LN, does not.
Even just taking those at the top in marketcap, ETH and XRP have nothing to fear. They don't really compete with BTC at all, in whatever guise.