It will likely require more resources. No keepalive on a backend connection also means you need to potentially perform another TLS handshake for each request -

It will likely require more resources. No keepalive on a backend connection also means you need to potentially perform another TLS handshake for each request - which is immensely expensive. The cost of idle TCP connections compared to that is pretty small (maybe 100kB RAM per connection, and no CPU cost). I agree that there can be drawbacks - e.g. Nginx trying to use a keepalive connection while the backend already closed it due to badly aligned timeouts. That's a potential availability issue which can only be avoided by careful configuration. Client not supporting keepalives shouldn't be an issue, since Nginx is the client here.