You can make an economic argument for or against cloud in practically every IT domain, but in HPC the case for on-prem is really compelling; none of the cloud networking/resiliency value-add is relevant to batch workflows, and costs per core-hour are only remotely comparable if you use spot - which is itself a major compromise.
The only real advantage cloud has for science is object storage, which is genuinely a much better idea than trying to manage your own long-term archival storage.
If I were independent I would recommend people buy and build on-prem clusters and shuffle data out of fast scratch into Glacier, but other than that just don't worry about cloud until price pressure kicks in and we are down to 1-2 cents per core-hour on-demand.
I'd love a role where I can say these things non-anonymously, but the salary for such a position would be at least 50% lower than working for a cloud provider. Keep that in mind when talking to your supplier - we may not believe the pitch ourselves, but making it is just part of the job.