BC and Sharepoint are going after 2 markets. Last I checked, Sharepoint was developed for intranets which is a different market than DW has ever gone after. I don't think DW could ever or will ever be able to compete with the suite of options offered by Sharepoint. The collaboration tools are what it's there for.
And on the BC side I don't think that will push DW developers away either. There are more sites made with the big 3 CMS' individually than there are with BC. Combined it's even farther from a fair fight. The amount of sites made with Wordpress, Drupal or Joomla far outweight Adobe's hosted solution that is only available by subscription.
DW will continue to be relevant so long as there is development. And it's not just DW, there are plenty of emerging web tools out there that are giving DW a run for its money. As for your clients, just tell them you know how to code, that will go farther than any tool you use.