Sending to 1,000,000 subscribers costs $55,000+ per year on MailChimp and other premium services. A cheaper option with the same deliverability would be extremely attractive to anyone who doesn't like setting their money on fire!
"I could switch from A to B and save $100 a month". Each human being in your startup costs $5000 - $10,000 a month in salary excl. benefits. Saving $150 a month on mailchimp is chump change.