if no new ppl use bitcoin and the existing people who use it start using LN more then yes basically :) But its not going to affect BTC much, the market cap may shrink, but the incentive is still for miners to be honest regardless of how small the security budget is. The only disparity right now is the people who claim BTC can be a world reserve asset, but this will not be true unless the security budget increases massively, which i doubt it will. So basically BTC is "just" going to be peer-2-peer money, we will still have fiat and crap money, BTC will just be something extra we use every now and then. It will be complimentary to the financial system rather than outright replacing it, but lets see