Note that this is failure to deliver on time, not failure to deliver full stop.
It's also important to note that the Hallmark of non-Agile teams is de-scoping and under-delivering. It's easy to deliver something on time if you switch your delivery goals and remove/half-bake features to technically meet requirements while not meeting requirements.
Things should be put into perspective. The cost per user is actually the fixed monthly cost of operating an instance divided by the average number of active users.
In the discussion you linked to, there's a post on how Lemmy.ml costs $80/month + domain name to serve ~2.4k users. If we went through opex/users metric, needlessly expensive setups with low participation would be a justification to ask for more donations.
Regardless, this is a good reminder that anyone can self-host their own Lemmy instance. Some Lemmy self-host posts go as far as to claim a Lemmy instance can be run on a $5/month virtual private server from the likes of scaleway.