I think we should aim for two things - resiliency so that a single failure doesn’t take out something essential, and a reduced dependency on individual goodwill. We already have resiliency on the discovery service, certainly won’t reduce that resiliency, and should move the relay registry to the same service. That’s really it as far as essential services are concerned, any other things are conveniences but don’t affect people’s usage of Syncthing as such.
For those I think the best choice is paid hosting from a well respected cloud provider. Outages with those should be measured in minutes, and there are people looking after stuff twenty four by seven. I currently provide hosting for these, because it’s easy and free and works lovely when it works, but it doesn’t make sense to depend on me for this forever. Likewise I wouldn’t want to “just” add more volunteers to the mix.