One reason for offering both would be if the loss of customers due to the switch does not exceed what they save with the switch. I'm not sure of the differences in manufacturing costs, but for many years every manufacture has been very concerned about getting better fuel economy to reduce CAFE fines. Shaving just 1 mpg can result in huge savings for a manufacture and the Hurricane is predicted to get 1 - 2 mpg better than the Hemi.
In 2025 the CAFE standard for small trucks will be just over 39 mpg. If a manufacture's sales in the small truck category does not average above the CAFE standard they must pay a fine for every vehicle that fails the standard. For 2025 the fine will be $14 for every 1/10th of an MPG the vehicle is below the standard. So around $2600 will be added to the cost of a 20mpg Ram costing the manufacture/customer millions annually. A Ram that gets 21mpg vs 22 will save the manufacture about $60 million per years in fines if they sell half a million units.
This source says the Hurricane is more efficient to build so the savings for the manufacture may be significantly higher.
I think the current 5.7 hemi will be gone forever. However, the current Hemi name was a fantastic marketing stunt so I wouldn't be surprised if they don't look for a way to carve out a dome in the combustion chamber of some motor so they can revive the Hemi name. I also predict many claiming they will only buy a V8 will change their tune when when they get tired of being passed by a Ram with an I6 twin turbo engine with more HP and torque and there is nothing they can do about it except make a lot of noise.