A single machine will have substantially less risk of breaking down than a complex setup.

Being pretty disingenuous here.

Single machine breaking down means an outage. Complex system breaking down means no outage. Again if you are running a toy site then sure go with the single machine.

And redundancy is very easy to get right if you are using something like AWS or even DigitalOcean. Provider based load balancer + App Tier + Multi Master database like Cassandra.