When you use that URL, Google also sets a cookie value that prevents the redirect to Google Photos even when you enter picasaweb.google.com in the address bar.
Google+ Photos has constantly improved last year, while Picasa Web Albums ** longer added new features. It's obvious that Google Photos is an upgraded version of Picasa Web Albums, which will be discontinued in the future. There are still a lot of cool Picasa Web features that haven't been added to Google Photos (Creative Commons licensing, photo mapping, featured photos, search), so let's hope that Google won't retire Picasa Web Albums until Google Photos adds these features.