My recollection when trying to find a Google Reader alternative was that there was no free alternative. Everything at the time that I found had been a premium alternative to Reader before it shut down.
But that is likely part of why reader shut down. Depending on how often it's pulling and how often you are reading, that isn't free to run.
Especially if you don't even use their app to read your feeds, you may never be able to see an ad.
Would rather pay for it personally so I know it's there.
But I have far more confidence in something at least making sense for the company to keep running if it is something I pay for vs something that is just given away for free.
If I am relying on an online service, while paying for it doesn't guarantee it being up it's a safer bet than a free one.
The server itself isn't free. Sure if they are smart if multiple people are subscribed to the same feed it only fetches once, but just the constant fetching would eat up resources. Then storing that data, you retrieving it yourself, etc.
https://blog.newsblur.com/2013/03/17/three-months-to-scale-n...
https://blog.newsblur.com/2014/03/13/google-reader-announced...