LN is a solution, but it is not fully decentralized. How this will be adopted depends on what the general public thinks is more important: Scalability or decentralization. If there is a way to trust a third party, it might be a solution. LN is not fully developed now, or at least it isn't easy to use now. When wallets start adopting LN, we will see how the community will respond.

I don't see Bitcoin going to zero as it is the holy grail of crypto. There will always be people interested in owning some Bitcoin even if the product can't compete with other protocols.

At the moment this is indeed one of the big downsides. This will probably change once crypto is adopted by the general public. Coins like Bitcoin have a big chance not to be adopted massively because a big part of the coins is in the hands of a small group of people. Other coins will come and replace bitcoin with a better scalable product and a better spread of coin holders. This will not mean that Bitcoin will disappear, but it might have to be adapted to satisfy this demand. The lightning network might facilitate this.