A multi node system tends to be less reliable and more failure points than a single box system. Failures rarely happen in isolation.
You can do zero downtime deployment with a single machine if you need to.
Just like a lot of problems exists between keyboard and chair, a lot of problems exist between service A and service B.
The zero downtime deployment for my PHP site consisted of symlinking from one directory to another.
Honestly, we need to stop promoting prematurely making everything a network request as a good idea.
But how are all these "distributed systems engineers" going to get their resume points and jobs?
A multi node system tends to be less reliable and more failure points than a single box system. Failures rarely happen in isolation.
You can do zero downtime deployment with a single machine if you need to.