This is my personal opinion about this topic! Again and again I stumble about this and now I want to share some of my thoughts about this topic. I want to share this from a point of view, when I was not a SAP Employee - I worked at a SAP Partner company - because this best reflects the view from the outside.
1. SAP "wants" us to use SAPUI5
I don't think that "wants" is the right word - the right word is offers! In terms of: hey now even included in your license and open sourced (because you wanted it so). Look - we build sooo many Fiori apps with it - and put in some great enterprise features (right to left, accessibility, translation, ...) and it is responsive. So if you want to build Fiori-like "apps" - there you go - this is your technology to go! This is what I understood.
I was very very very happy that it was build with open standards. But the main point for me another thing - (because I worked with Sybase Unwired Platform and SAP Mobile Platform) OMG an open data protocol!!! OData FTW! I could use ANY front-end technology and consume SAP data - the protocol is REST based - a dream came true. And so I did - yes I played around with Sencha Touch and OData, I used data.js and used it in a standard HTML5 application. We played around with an iOS application we already had and consumed the data. I feed the data into d3.js. I prototyped around with SAPUI5 and I have build apps with it. Brilliant, so I could choose whatever UI technology I wanted.
I always had the feeling, that SAPUI5 was meant for B2E applications - and building many of that - and they should look and feel the same and I can theme the apps. I can use it when I want to make my applications SAP like - so that the user thinks the apps are all the same and everything fits nicely into the Fiori launchpad - great if you want to build partner apps. In my ex company we won a SAP Pinnacle Award - for a native iOS app, no SAPUI5 in sight - but we have received an award, so SAP did not "want us" to use SAPUI5 ;-)
2. Is it the right UI technology for everything?
At my ex-company I was dealing with mobile applications and web apps. I always knew that I get the best user experience when I write a real native application and if you want it to look like in a very specific way with awesome UI --> you would not do it with a write-once-run-everywhere HTML5 super-styled wrapped (phone gap/cordova) app - you would write the apps for the specific platform and it should feel like it was written for the platform. SAP offered us the SAP Fiori Client that I could enhance the user experience for the Fiori apps, but I was never told to use SAPUI5 it for everything.
The same discussion goes on with: why was the new SAP website not build with SAPUI5, or the UX Explorer (eat your own dog food), or a useful internal app - why Angular, or the other way around why do we even use SAPUI5 and not Angular JS itself. Back to the past - SAP did never tell me to build a website with it, or to build something tiny and small with it (a widget) - or to enhance parts of a website with it - or to build something super-specific with it.
And yes maybe the UI in a mobile app was not SAPUI5 but do you know if it uses Gateway with OData or the SAP Mobile Platform, was the API managed with Apigee, was HCI involved in getting the data from different sources, was it maybe wrapped with SAP Mobile Secure, did it maybe use a HANA backend or some of its features (predictions, text analytics, ...) - are some backend parts maybe hosted on HCP - who knows? So yeah, maybe other SAP technology was involved which you don't see, but in the end it made your life simpler.
Instead of arguing around why this and that was used for this and that - can we save our energy and instead look at the result - which is all what counts - that it was the right UI technology with the right user interface which makes the user happy? I stated something like - does Google use AngularJS for everything? Nope. They offer us something great we can use. Right, maybe AngularJS is not as important for Google like SAPUI5 is for us. But we have something with which we can build our day-to-day business apps with and we do so.
3. We missed great opportunity in not using SAPUI5 for this and that
Let's think about how projects are going: you want something, you look at the costs, you choose most efficient option. Yes this could mean reusing an existing native application which was written long ago and you pimped it up. Or you had outsourced it to a company which wrote a similar app. Or you bought the source code. Or you have some cheap internal staff which could do it (students, trainee), or you could outsource it cheaply. Or all you have is people with experience in this and that technology. Or it must look in a very very specific way (because marketing says so) and you go native. Or it maybe should even differentiate itself and should look NOT like a Fiori app. And no - I don't think - this is my employees view - we are not for example an event app producer. We normally build business applications. And I don't think that we can write an app from scratch "just" for an event in a certain technology. Yes it would be nice if this and that would use SAPUI5, I think if it would be possible in terms of time and costs and UI wishes and hundreds of other factors we would do it with SAPUI5.
Working at the partner SAP showed me over 50 Fiori apps, now hundreds of Fiori apps are released. SAP now showed that SAPUI5 is going big with S/4HANA. We have seen the Simple Finance solution. I was walking around at the CeBIT this year and I was impressed myself which cool apps and screens have been shown with SAPUI5. I used the SAPUI5 app on a Samsung Smart Watch myself. The SAP Web IDE itself is built with SAPUI5 in its heart. I don't need any other "proves" that SAPUI5 is great. I have seen great use cases for it, but I also know myself when I would use other stuff.
4. My conclusion
There is never every anything which can cover all different use cases. One-size-fits-all clothes also does not fit for everyone even if it says so. If someone creates something great don't judge it by the UI technology. Judge it by the experience and the value it creates. There are reasons why this and that was chosen. Use what fits your needs best with your requirements. Make the end user happy!