After everything said in #1654, why is 16-bit color still the default? Analysis there wasn't detailed enough lol?

It's better to just let go of 16-bit color. No parameter/option.
If you really need 16-bit color for some niche reason, it's better to implement a dedicated create16bitBitmap() method for external users, and let the rest of us enjoy true color tiles out of the box.

16-bit color should be regarded as having special usage in (micro-)optimizations, as a last resort when dealing with tons of large bitmaps at the same time (think gallery or photoshop app), and even then it's a big if. This library is nowhere near such reasons.

If you need a little more convincing, look at hill shading gradients in 16-bit color...
Is there a need for additional analysis of the pros and cons?