Subs were already an option since the world has decided not to deal with Iran and let it develop it's own nuclear missiles. Israel is simply taking the extra step ahead by making sure it can respond even after receiving a nuke on it's head. Long range missiles can prevent countries like Pakistan to threaten Israel with anything as well.
UN to hold off on separate Afghan bombing probe for now (Reuters)
from the article: Report: Israel seeking to upgrade its nuclear weapons capabilities