Maybe, but it would also cost Google almost nothing to set up a 301 redirect from Picasa URLs to Google Photos. I wouldn't expect someone like Yahoo or Microsoft to bother, but being a “good web citizen” seems intrinsic to the Google corporate identity, after all.
But that doesn't solve the issue that the photos on Google Photos are private by default, so Google would need to change the sharing to public on all photos that have external links, which would be a terrible privacy measure.
