Many companies which had a need for customer.io previously a) hired out very expensive people to build it to them (I have a couple of five figure invoices attributed directly to it not existing as an off-the-rack option last year) and b) can trivially go through several hundred bucks just having a single meeting on what to say in the emails.
You can, of course, code it for yourself out of duct tape and bailing wire, and that will pretty much work if you never need the non-technical marketing team to touch it.
I don't know, "other people" might have their hands busy, considering this prediction has been made about a hundred other apps on HN.
But I think you are underestimating how much effort it is to build something which businesses want and for them, ROC is more important then what a particular software/service costs. A business will happily pay a million dollar for an extremely simple software if that software can either save them 2 million dollar OR increase their revenue by 2 million dollars.
