I don't mind them having a free tier and tweaking their offering. But I think grandfathering in older accounts is nicer, and I wish it was easier to work with accounts in a technical role when you are not technically the client, but are acting for them in most ways.

e.g., I can set up their account and other technical parts after that, but I can't verify their mobile number (added since we started working with Mailgun), don't have their credit card details to put on file (same), and often end up agreeing to legal notices for them rather than asking them to complete signup (and stalling the project).

Mailgun also wished that you'd have signed up as a full paying customer when you first wanted their service. But they knew they weren't gonna get what they wished for either...

Never assume free tiers will last forever. Recognize they are often a way to entice people into the system. And that often they change as the business grows. Thus don’t build your business that depends on free tiers.