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 my opinion no because there are so many altcoin which are based on their products usecases and if someone makes product they have marketing plans too. Infact they will also implement LN in their platform. So altcoin will not gonna disappear and will remain in the market with giving competition to the BTC.