Email sending, web workers, etc. can all feed off of the same repository, running on different hardware, and have functions subdivided by DNS. "Monolith as services" is a super common pattern.
The closest thing I can tease out of this claim that seems like a significant practical criticism is that you need a resilient communications channel for talking from, say, core logic to an email sender--but you need that on one node too, if you aren't using a the right datastore for the job you're asking to staple your hand to your face no matter how many nodes your application runs on.
And these days it's pretty trivial to use a lot of AWS free tier stuff--or other clouds, but let's be real, AWS gives you A Lot--for much of what you describe. CloudFront is not my favorite CDN but they give you a bonkers amount of free transfer these days. DynamoDB as an object cache, if you don't want to pay for Redis. SQS is free to like a million API request a month and you can stick a lambda on the other end of it to handle offline/async tasks. And SES turns email sending into an API call.
If your concern is velocity, then cloud stuff is almost unassailable. If your concern is price, better cloud stuff is hard to beat, too. If your concern is portability, I hope you're already profitable and that it's worth your time to worry about it.