IM amused by some of the JS community acting like server side rendering and hydration is akin to discovering fire when they just brought back progressive enhancement from circa 2009.
Next week we're going to have a lesson on semaphores.
On a more serious note, there are a lot of places where we could take some lessons from the past to heart. (read stop reinventing the wheel). Permissions systems spring to mind... Something unix/ldap like would fit a lot of use cases and be much more clear that some of the awful things I have seen. Good database design needs to make a come back (and people need to stop being afraid of sql). Being able to let go of data, why do some people have so much trouble purging old logs.
I could go on but... dam you kids get off my lawn!