I don't know if Strong has been discontinued . But emailing Bona can confirm if it has been. I do know that Mega and Traffic are regular products and use them a lot. Mega with fortifier should be pretty comparable. Traffic would be a step up in durability and cosmetic appearance too.
Another option would be to use Basic coatings Street Shoe. my only gripe with Bona Strong is that it yellows a lot . It's not a very good looking water based finish in my opinion . For Bona finishes, check online with
www.floormechanics.com