Your comparison assumes that the alternative to backdoor Roth is a traditional IRA contribution. The (ab)use of backdoor Roth contributions is commonly done by high-earning employees who have access to a 401(k)/Roth 401(k) plan. In this situation none of their IRA contributions are actually tax deductible at all!

So the 'backdoor' Roth contribution allows such people to invest post-tax dollars w/ Roth benefits when the IRS income limits seemingly make it clear that they shouldn't be able to contribute in this fashion. Why does the IRS have income limits on Roth IRA contributions but then allow them to be so easily bypassed in this manner?

> assuming growth and tax rates remain the same.

Big assumption.