"But if you don't expand into other markets, you'll eventually die!" Sure, but that's not a bad thing. The purpose of a corporation isn't to continue existing in perpetuity, but rather, to maximize returns for its investors. You found a gold mine? Great, go mine it until it runs dry, then close up shop and call it a day. There's no need to spend half your profits on a wild-goose chase for other gold mines (blackberry investing in self-driving cars anyone?) - if that's what I as an investor want, I can just use my dividends to invest in other companies or startups that are aiming to do exactly that.
I'd rather invest in a company that sticks to the things that it can reliably excel at, and direct its profits back to me, so that I can then invest it in other companies that I trust in other fields. A company that is spending half its profits trying to be something it's not - that's my worst nightmare as an investor.