The purchase option for webassist extensions still exists. If you don't like the leasing terms you can continue to purchase extensions as you would before the subscription option was announced.
Lease and rent are synonyms. Car leases usually include the option to buy, but really you are renting for all intents and purposes, because usually by the end of the lease the car value is about the same as the payoff.
You are correct that the library lease doesn't include a buy option or discount, but that is because unlike a car during the lease period your software will actually grow in value and functionality and be newer and more functional when you cancel then it was when your lease began.