My company (MailChannels) sends email on behalf of more domains than any other transactional email provider (see https://trends.builtwith.com/mx/transactional-email/traffic/... ) because our customer base is hosting companies rather than individual senders. The whole transactional space has matured in the past year with SendGrid's acquisition by Twilio and Mailgun passing through a couple of different private equity…
Having a big free tier is just a growth hacking technique, and it worked: people know the product and developeed integrations.
But, all marketing tactics have a beginning and an end. If the market is mature, and it's ROI is not good enough or has decreased dramatically, it will end soon.