Oh, this is an easy one.
I would like to be simply one of the biggest producer and distributor of toilet paper.
Yes, toilet paper! WOW, it is a wolrd mass businnes, every single human in this world need it. and even not-humans sometimes. That's easy, 7 bilion of potential customers, no way to run out of them, no risk that science could damage you, they are trying to let people living longer and so is even better. If you are a coffin seller you can worry about all this long lives and about immortality, but if you produce toilet paper, you can bet that humans will never stop to use it for a looooong time.
If one day they'll invent some sort of alternative to paper, well, you will be rich enough to adapt your factories and your product, so you will mantain your name and your customers.
Easy. Simples rolls of soft and resistant paper.