It is more than that - lead and ethanol have other properties that engines that use them need to handle. Lead also acted as a lubricant and parts designed for engines that assumed lead fuel were designed with softer valve seats - switch to unleaded with otherwise equal octane and your will destroy the engine. (though experience shows that unless you were driving your car on a race trace most cars worked fine for longer than the car lasted). Ethanol will destroy some forms of rubber and so you need to use different seals in some parts.

TEL was patentable, but those patents were long expired before there was a big push to eliminate leaded gas.

Also, TEL being patented by Dow (which isn’t an oil company) actually was a reason oil companies would want to use an alternative, if possible. Why would they want to pay Dow to use a patented product, all else being equal?