The F-Droid repos are provided by redundant mirrors: https://f-droid.org/en/docs/Running_a_Mirror/

If this is the hidden master server that only the mirrors talk to, then it's redundancy is largely irrelevant. Yes, if it's down, new packages can't be uploaded, but that doesn't affect downloads at all. We also know nothing about the backup setup they have.

A lot depends on the threat model they're operating under. If state-level actors and supply chain attacks are the primary threats, they may be better off having their system under the control of a few trusted contributors versus a large corporation that they have little to no influence over.