Sounds good in theory, but what if your company decides to reduce investments in the spreadsheet product and instead transfer the product team into a much faster growing product segment, e.g. SAP integrations?

Then the perfectly qualified PMs start from square one again.

IMHO it's an essential skill for a PM to familiarize him/herself with a new domain in short time. It also offers cross-pollination / the chance to do things different. Imagine Steve Jobs coming from a pure IT background instead of liberal arts / calligraphy -> Apple wouldn't have been so different any more.