I think this a point a lot of people don't understand (or they think he's just being glib when he says that). The problem the development team faces isn't primarily money or team size at this point (although they're always looking for talented staff), it's time.
They have a dev cycle that keeps patches coming out consistently every 3.5 months, and the stability of that process depends on many hundreds of people's work coming together at just the right time on a consistent basis and coalescing into a single product launched simultaneously across multiple platforms in multiple languages. The logistical complexity of doing that for a once-and-done game is bad enough (consider how many of SE's other projects have seen massive delays), but to keep on doing it on this kind of rapid schedule like clockwork means they are like a machine.
As he points out, adding new races doesn't only have a one-time development cost (although there's definitely that), it also has an on-going upkeep cost, and this added upkeep can't interrupt the existing development cycle and flow. So even if the CEO came to Yoshida today and gave him a proverbial "blank cheque" to "get it done, no matter the cost" the big issue is how to re-arrange all the logistical pieces to keep things working on an on-going basis. Some of that may involve new tooling that allows them to work more efficiently. Some of that might involve a new spread/distribution of content across the patch schedules to balance workload. Some of that may involve a new approach to managing the team that better utilizes outsourcing or other techniques. But it's not just a matter of money and people alone.
As you said, it's important that people keep giving them feedback and pushing for the improvements they want, as that definitely helps the prioritize and keep considering ways of accomplishing the goal. But the idea that they're woefully underfunded and a sudden infusion of cash from SE would suddenly mean all the things they currently say they can't do would suddenly become possible is just a fantasy. Yoshida's number one job as producer is maintaining the steady march (the entire assembly line) that keeps the content flowing at a stable speed, and there are only so many disruptions the project can weather without disrupting the pace. (Of course, his job as director is to create a product that makes customers happy, so everything has to be balanced. That's why the feedback does matter, and why they took the time to write the letter.)