Churn of the user base could be playing a role in this, but I think it may not be too significant. In Europe there are multiple universities with HPC masters, which provide new users/devs to HPC. I worked with HPCs in New Zealand, and now I am doing the same in Spain. We hire multiple people from other HPC centers in Germany/UK/Italy, and equally lose people to those sites.

I think the field is actually increasing with AI, digital twins, more industry projects (CFD, oil, models for fisheries, simulations for health diseases, etc.).