Yes! You'd need to press ⌘B to enter bold mode, and then begin typing.

It's ever-so-slightly more effort, to be sure. But, I loose so much more time trying to add non-bold to the end of a bold section, or vise versa.

I can learn to use systems which behave in repeatable and understandable ways. Once a system starts making decisions for me based on information I can't see—like invisible markup tags—everything becomes much harder.

The alternative is of course to make the markup visible, but for writing prose, I appreciate the clean look of WYSIWYG editors. Visible markup can feel messy.

Text color, background color, strikethrough, superscript, etc.? If I'm typing in a block of red text, is my new text black unless I tell it again to be red? If I'm editing a superscript, do I type normal text in the middle of it and then need to select it and make it superscript afterward? Editing text that was highlighted with a yellow background?

It might be an easy toggle for bold/italic/underline but there's a lot of other text formatting that would either need to behave differently or would become a huge pain. Personally I'd rather keep it consistent.