Selling the value of Rise & a quick poll

Some 3-4 years ago i had the same issue at hand, i then started doing rise courses, motivated by the rather high cost of the Articulate360 suite and adding responsive design as the new value. Today it is a recognized standard in a +100K company.

RIse is included in the Articulate360 suite. If you have that it, it is less value to only use storyline for most basic stuff.
Its like buying Adobe creative suite and only use photoshop. 
You can add storyline objects to your rise course. There is strength in finding the balance in combining custom interactions from storyline and rapid course development, all in rise.
If you create that type of example for your stakeholders,  i think it can show the Suite has untapped potential and then they will leave the decisions on what works for training up to your discretion from there on. Still hoping they get involved in all the the course design specifications of course.

But in the end, Rise is true responsive design for any device, Storyline is currently not.
Combing the two is something fairly unique in value.

I also think your L&D people are right about voice etc.. But not in the sense that it excludes rise and the value that the tool brings to the e-learning development process and the user experience. Rise is growing in features and the expectations of the future with rise is why we are just about to re-new for a multi year Articulate360 contract. some 60+ licenses.

So in the answer i cant say 100% rise nor can i say 100% storyline.
But i can say 100% Articulate 360 suite.

Hope this helps your situation.