No, I'm not assuming that. The quantity sold will decrease, of course, if the price increases. That is a hidden cost paid by prospective buyers who must now do without the good, complementing the more visible cost paid by those who continue to purchase the good at the higher price. A company's profitability (ROI), on the other hand, is relatively fixed—in the end all investments tend toward zero economic profit, or equivalently a level of accounting profit equal to the opportunity cost of the investment, assuming even a moderately competitive market. Higher-than-average profitability invites competition, while on the opposite side if profitability is low then investors put their money elsewhere and the business closes. There is some variation due to factors like brand recognition, other natural or artificial monopolies, and risk, but generally speaking a corporate tax won't be paid from the company's bottom line as most people imagine. There just isn't much flexibility in that area to begin with.

> Exxon not paying much/anything in taxes, and $B spent every year globally to keep the supply of oil flowing.

This is not a subsidy to Exxon, it's a subsidy to the people consuming oil-based products. If you want to stop subsidizing oil, fine; but that is a completely separate policy decision from where to levy the tax. Anyway it's not like you're proposing to tax Exxon specifically for the cost of keeping the oil flowing, which would defeat the point of the subsidy. A corporate income tax would spread that cost across all products, even ones that have nothing to do with oil, and be paid primarily by people who spend most of their earnings on corporate-produced goods (i.e. not the rich, who tend to spend a larger share on investments).