Seems to depend on industry and/or region.

Most of our customers have a hard requirement on ISO 9001. Many on ISO 27001, too. The rest strongly prefers a partner having a plan to get ISO 27001

Pff… You wish. Depending on the sector you are in, ISO 27001 can either be a hard requirement (either directly or through national standards built upon it, like the Dutch healthcare NEN 7510) or completely irrelevant. If this company needs it, you can bet their customers need it — usually because they in turn are required to do so because of regulations.

You must be looking from US perspective. In EU I don't think I've seen any SSAE provided or wanted, and I've seen a bit of medium to big industry.