Having 1/3 of the "country" out of their hands is a pretty important issue. Best believe there will be a war if Somaliland doesn't bend the knee like they expect us to. Direct war isn't the only possibility that can happen but a stronger Somalia would have a higher chance of foreign governments and organisations respecting their so called sovereignty and they can isolate us through various means diplomatically and economically. Things like the Berbera deal and other foreign investment would probably not stand any chance, direct foreign aid to Hargeisa would be out of the question, they can ask foreign airlines and shipping lines not to operate in Somaliland, a ban on trade etc.
A good example of this is the Northern Turkish Republic of Cyprus that is effectively embargoed internationally through Cyprus who has lobbied successfully to the UN,EU, various International Organisations and countries. They can only manage to stay afloat due to Turkey's support, we don't have a patron so you can see why this would be even worse for us. This is what you are effectively looking at in the face of a stronger government down south. Don't be complacent sxb, no doubt they are in a weak position today so they have fewer options but when the tables turn, they won't hesitate to cripple us.
https://en.wikipedia.org/wiki/Embargo_against_Northern_Cyprus
Having a peaceful Somalia doesn't necessarily mean that it will be a strong state. It'll be 50+ years before Somalia will have the ability to put an embargo on Somaliland. You're really over estimating their capabilities here.
As long as Somaliland and Somalia are stuck in this unsustainable union both sides will focus on tearing one another down in order to somehow gain an advantage over one another. All that does is hold them back and keep them weak and susceptible to foreign meddling. As long as this remains there will be no threat of one state taking over another. The real threat is that both governments sabotage each other and send us back to weak central authority and clan rule.
This is why I support a 2 state solution. It will end the animosity they have for each other and form the basis of a new relationship of economic and security cooperation.