I spent several summers working at Los Alamos National Lab in their HPC group on I/O, among other things. Supporting legacy codes was a requirement, even if it

I spent several summers working at Los Alamos National Lab in their HPC group on I/O, among other things. Supporting legacy codes was a requirement, even if it meant that we couldn't explore obvious and important optimizations. I remember an anecdote from the division lead, that the reason a lot of legacy code is never replaced was because of certification. Codes are designed to simulate some critical system, and it takes years to trust the result. So any change that forces a multi-year validation process was a non-starter. My understanding is that the code modernization and co-design efforts that are a part of the Exascale initiative are changing this.