Slavery, monarchy, and arranged marriages were much older institutions that ended.
Big LOL on equating slavery to patent rights. The patent holders are the ones getting screwed by a tiny amount of monopoly time granted to make money off their invention -- just 20 years. If you subtract the time taken from patent filing date to bringing the product to market, including marketing, say 4-5 years, the monopoly is only for 15 years. And the first 3-5 years, you won't have much sales because nobody has heard of your product. So the effective monopoly is only for 10-13 years, not a long time.
There are plenty of very ordinary businesses like furniture sales and fast food franchises that make money for decades.
So how exactly are patents free gifts? Take cars for instance. The total revenue from new cars in the US alone is tens of billions, yearly. But because of poor patent laws that grant you monopoly only during the initial phases of the selling an invention, when sales of cars were relatively low, the inventor of cars gets nothing, whereas the patent freeloading capitalists, who wait until someone else has worked to make a product popular, then enter the market and seize control of the market share by outspending the little guys, and make all the money for centuries and kick the inventors out of the market.
In fact, like copyrights, inventors should get an additional 20 years where they can charge a 1% sales royalty to companies manufacturing products that use their invention. The start date of royalty should be chosen by the inventor. If the sold product uses multiple patents, the royalty is split among the patent holders.