You make a valid point, there is nothing stopping the Hells from go on their own. The mafia is in turmoil on account of this war and have become weaker. The same was true with the Hells when they were having their war with the Rock Machine. They were weakened considerably but they made a comeback.
There is another factor to consider is the Ndrangheta, have they made a alliance with the Hells or other mafia clans from Quebec?