I don’t know what the per-unit profit is. I’ve only found production numbers, and those aren’t huge. I would imagine a company takes a somewhat broad look at a product line when deciding where and how to invest, including paint changes.
The majority of 4Runner buyers want a 4Runner specifically, from what I’ve seen. Not many 4Runners are sold to people who simply stumble across it in their search for a suitable vehicle. Toyota knows that. As a result, they price them according to that specific demand. Just business, in my view.
You're very much correct here! It explains the very long redesign cycle. I can't think of any other vehicle that has gone so long without a redesign besides the Tundra. Maybe some FCA cars? Difference being that Toyota clearly understands its market, because they're selling just fine today.