There are plenty of transferrable skills you get from being an expert something that gets made obsolete by a similar-but-different iterative improvement. Maybe you're really good at implementing ideas from papers, you have a great intuitive understanding of how to structure a model to utilize some tech within a particular domain, you understand very well how to implement/use models that require state, you know how to clean and structure data to leverage a particular feature, etc.

Also, being an "expert in LSTM" is like being an "expert in HTTP/1.1" or "knowing a lot about Java 8". It's not knowledge or a skill that stands on its own. An expert in HTTP/1.1 is probably also very knowledge about web serving or networking or backend development. HTTP/2 being invented doesn't obsolete the knowledge at all. And that knowledge of HTTP/1.1 would certainly come in handy if you were trying to research or design something like a new protocol, just as knowledge of LSTMs could provide a lot of value for those looking for the next breakthrough in stateful models.