If you go to a web site that does A/B testing, you're going to get a set of instructions about how to install it on your site, and then it's up to you to figure out what changes you should try. You now how more work to do, and no better idea about what you should be trying.
If you really do have a way to replace a full-time marketing person with an automated web site, build it. It will be a gold mine.
Think through the pricing model of the hypothetical web site you propose in the last sentence - that is my point.
If a web site charged $500 and was very easy to implement, I could do it as an experiment and not lose much if it failed. If it succeeded, that web site could charge me again and again to keep doing it, and I would be happy to pay.
But if I have to spend $8000 and several weeks up front, I'm going to be very cautious. It doesn't matter if that money and time are spent hiring someone or purchasing and installing an automated tool.
"I'm not saying that there isn't a person out there who can revolutionize my marketing in 20 minutes" - I'm saying you wouldn't pay them to do so in line with what doing so would/should be 'worth' to you. There is no market to pay to revolutionize your marketing in 20 minutes, at a pricing that is based on what it should be worth for you to do so. Doesn't exist. You wouldn't pay it. Imagine what "revolutionaizing your marketing" means, image what "revolutionizing your marketing" would be worth to you - do you have that figure in mind? Now imagine paying that to someone for 20 minutes of their time. You wouldn't. Neither would anyone else. That market doesn't exist.
Markets can remain irrational longer than B can remain solvent.
There's pretty easy proof of this. You would spend weeks of time, $8000 in recruiting expenses, trying to get a web marketing manager who can increase your conversion rate.
But if there is a web site that you could allow to A/B test some change to your layout - that its operator somehow comes up with very quickly or even programmatically - you would not give them $5000 for a few minutes of work. It doesn't matter what the 'results' are, or if it's in escrow or tied to the results: you just wouldn't. You would prefer to spend weeks of time and double the money on a web marketer who doesn't work as well.
And for that reason, such sites just don't exist.
There is no place on the Internet you can pay someone $10K for 20 minutes of work, even if those 20 minutes are objectively quantifiably better than the results you would otherwise spend 6 months and $100K trying to achieve.
Human psychology just doesn't operate like that.