It already has ended well for KSA. 85% of Yemen is in the control of the Arab coalition and Yemeni government/Yemeni army loyal to the government and allied tribes.
KSA has not invaded Yemen as in invaded the country using a massive ground force. What KSA/Arab coalition did was to bomb (at the beginning of the war) all the strategic areas controlled by the Yemeni army and Houthi's and afterwards moved into Yemen with a LIMITED number of soldiers to conquer/liberate key areas of Yemen such as the oil and gas fields, the main and most important harbors, the economic heart of Yemen (Aden), most of the agricultural lands etc.
This is the most recent map of Yemen:
Nowadays as I wrote to you already, most of the fighting inside Yemen is not done by KSA/Arab coalition but mostly the Yemeni army that has regrouped after 2 years and which KSA/Arab coalition helps finance (along with the US btw) and help train.
At the same time we work together to combat AQAP which Trump has intensified the fight against which is a very good thing and supported strongly by KSA.
Hadi was always a transitional figure and he is irrelevant other than being the symbol of the legitimate rule in Yemen. If he dies tomorrow this will not impact the war other than finding an alternative.
KSA/Arab coalition tried to deal with the Houthis through reason but it is hard to deal with them any other way when they only understand violence and power. How come do you think they managed to conquer all of Yemen (almost) pre-March 2015 back in September 2014? By giving out flowers?
I gave you a link so you can read about that Houthi takeover which happened before KSA had intervened.
https://en.wikipedia.org/wiki/Houthi_takeover_in_Yemen
And the Yemen war is complex and KSA/Arab coalition have committed mistakes there is no doubt about it but it would have been much worse had we allowed Houthis (who are incredibly incompetent people) to run Yemen and create a terrorist safe heaven and potentially an Iranian base/Hezbollah next door who the longer we did not act would become harder to defeat. Instability in Yemen is not good for KSA either although most of Southwestern/Southern KSA is quite isolated from the remaining part of the country. In any case KSA has nothing to gain from a unstable Yemen.
The war has actually not been that expensive. Had it been incredibly expensive KSA would have stopped it a long time ago especially in times of budget deficits.
There will be no war between KSA/GCC and Iran has no party will gain anything from this other than foreigners. It would not be a cheap war, never said anything about that, in fact I never spoke about any KSA/GCC/Arab-Iran war but it would be more expensive for Iran as their economy is much worse than KSA's let alone the combined GCC economy. Don't forget that the GCC has sovereign wealth funds worth USD 3 trillion alone. That is a big amount of money.