A single bare metal server is more reliable than most people think it is. Complexity adds a lot of overhead and layer after layer that could possibly fail.
This is not true at all. Electricity prices are crazy and hardware is a lot more expensive for us (not affiliated with DO, I run a PaaS service) than it was 2 years ago.
Wow, that's a surprise! I love Postmark and I am using it for all my products. This kind of feels wrong though, hopefully it won't go downhill from here.
Please correct me if I am wrong, but you can simply snap install with a --channel, which could be a specific version. This way it is not auto updating, since it's on that specific channel/version.
Rails will handle "thousands of comments" just fine. If you are hitting scaling issues with a midsize Rails app you probably could look into how to optimize.
Throwing Mongo at the problem is not really a solution.