For most designs, I would agree that the “Send” control ideally should be visible in the default window size . . . but in the context of all systems with windows, there are many different devices, operating systems, and screen sizes, and many of them have the operating system determine what the default window size will be, and the operating system will usually not know anything about the Send control.
Often an operating system will allow a program to set its own window size, so the program itself might try to calculate a minimum good size when it gets launched, and request to be shown at that size. On the other hand, not all operating systems allow that, AND some users may complain about programs that start their windows out large or partly off-screen.
Some programs may be set up to scale their display to compensate, but if they’re given a small size, and they scale to show their Send button, it might need to make everything very small to do that.
In short, yes that’s a good goal, and can often be arranged for most cases, but there are cases where it doesn’t work out, especially when the developers assume a different screen size from the one someone uses, and/or the operating system gives a program smaller window size than the developers expected.