That's probably the reality of the situation. The number of people using the basic HTML mode might be vanishingly small, even if the user base is very vocal. Also when you have as many users as gmail, even 0.01 percent is going to be millions of users.
There may be a business case for someone to make a gmail client, or a competitor for those relying on the basic HTML mode. What is a rounding error for Google could be a pretty compelling business for a much smaller company.
Honestly I think 0.01% would be optimistic. Most people that don't like Gmail's fancy features just use third party clients. Unless Google decides to start blocking third party clients, which I could imagine them doing.
There may be a business case for someone to make a gmail client, or a competitor for those relying on the basic HTML mode. What is a rounding error for Google could be a pretty compelling business for a much smaller company.