Not sure why some people are calling it dead already.
1. While he's a core dev, he's definitely not the only dev
2. Lightning has never been perfect. But while it technically works, it's simply not ready yet. There's a reason why I never recommended it to normies yet as of yet. But, let's not forget that —
3. Software can be improved
How much impact can one core dev have on the project if he pulls out? It will probably allow for other devs to put forward different likelihoods of how to go forward. Maybe other devs can start finding workarounds or solutions. It may end up being one temporary solution to another but if it works Lightening users will not complain too much.
Lightning's fucked, but not dead. I don't see why it would be. There's just too much support for the network, and they're filled with capable devs that could take from where Riard will leave. Plus at the end of the day he's not the glue that puts everything together, long as there's people who are willing to improve upon Lightning Network cause just as what MK4 has said it's far from perfect, it will remain functional and pretty much alive. In the event that it does die, I don't think it connotes to anything other than previous efforts about Layer 2 solutions being moot.
That cannot be denied, Lightening does have a lot of support and it is used more commonly than before therefore it is going to continue. Some of the noises about it becoming a relic of the past are somewhat premature but they will not stop until there is improvement to show it is capable of functioning