You can disagree all day, it won't change the facts.
"Investment" is not limited to adding employees directly. If the government takes an additional $12,500 from you in your hypothetical example, that is $12,500 you (and millions of other entrepreneurs) no longer have available to purchase equipment, computers, pencils, advertising, etc.. That is why whenever tax rates increase - the amount of cash available for "investment" decreases- and unemployment increases because of it.
This is where I get a bit confused.
Again, if it means more hires, they should be shouting "cut taxes" from the rooftops.
I don't think it means anything when you cut taxes. If I have a side business and you cut my taxes, I'm not necessarily going to hire people just because I can. I'll pocket the windfall, perhaps buy some more stock...maybe.
It's not a guarantee.
Just like raising taxes isn't a guarantee that there will be slower hiring.
I will agree that if you have less money in the hands of business owners, they will LIKELY not hire more people but I doubt they will start firing people based on having to ante up another $5K a year. Maybe but I don't think so. They still need to compete with others.