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.
Toggling bold should function identically to toggling caps lock: if I press the bold button, any letters I type should be bold until I press the bold button again. Caps lock does not disable itself when I click into a non-all-caps block of text, and neither should bold.
This would be annoying sometimes, but it would help far more often than it would hurt. For some reason, however, nobody does it this way.