That's a hell lot of things to change, break, go down, lose your data, start charging ridiculous prices, be discontinued, etc... I'm all for SaaS and not re-inventing wheels, but I'd be nervous if I had to rely on so many external dependencies.
I can understand that competent sysadmins can recreate heroku's underlying hosting, but there's still a decent switching cost (especially if your whole team is accustomed to the heroku way). I've consulted with several companies that have 5-figure a month hosting on heroku that they can't migrate after all their investment.
If SendGrid changes my plan (or somehow degrades), it's a simple config change to send my email through another provider. A more apt comparison would be if your mail provider used a custom ReST endpoint instead of SMTP (say, as MailChimp does).
Sometimes though you can make do with something significantly simpler if you restate the problem slightly and move the pieces around.
You might be able to do the core of the job with a cron job and some command line tools rather than the fancy SaaS platform with graphs and everything.
Building on top of the fancy SaaS APIs and then trying to scale down when you want to move away is a lot more difficult than starting with the simpler approach and then scaling up when you really need that extra functionality.
Believe me, I've done it in both directions.
When you are raking in profits, you'd better use some of them to secure your business and make it independent of such risks. Which is the point the blog post seems to be trying to make.