My personal opinion on that:
Classes/Types
That argument from people (ABAP, Java or C#) that "types or classes" would help them to learn JavaScript is totally valid - I am also looking forward to ES6 class, but still - I need to learn the original prototype stuff. On code completion I totally agree, only good editors are here to help. And undefined is not a function is mainly due to poor error handling or accessing non available deep nested objects methods. That is not a language fault - ok you could see some errors before when it would compile. But wishing that the language would be this and that is like: oh Python, I constantly do { and ; could a Python dialect fix that for me?
Maintainability 1
Internally there are a loooooooot of people creating SAPUI5/OpenUI5 and using it to create a bunch of apps (now with S/4 HANA a big bunch). To get all non-JavaScript people on board with JavaScript + SAPUI5/OpenUi5 was sure pretty hard enough and now adding additional stuff like TypeScript is I think a bunch of work for such a big company and I believe out of question. For our framework we are our best customers, adding additional complexity would be hard. And since we are supporting lots of older browsers, you should be able to read your transpiled code (extra work).
Maintainability 2
Another thing is when we would handover apps or framework stuff to the maintenance team, how much languages do they have to learn in the next few years? Now maybe Typescript is cool, before CoffeeScript, what about Dart - you could not afford that in such a big company to support every "hip" language dialect, we should close to standards. Concerning this, yes I would call jQuery a "standard".
Getting rid of jQuery
I am not deep into how we use jQuery in the framework, but for me as an enduser - yes provide me an equivalent for $.ajax ;-) JQuery messed up with Promises, but as John said we have our ways around this.
If I would write var Button = Control.extend(... or Button extends Control in future - hm more or less syntax stuff. SAPUI5 is a big framework, getting rid of jQuery would not make it that much smaller.
Summary
If you are your best customer with a framework and have a long maintenance period and support for a bunch of browsers - staying close to JavaScript only and jQuery is not the worst decision. And yes -even if it is not that cool to wait until certain stuff has been declared as standard - we have a large customer base to support. SAPUI5 is an enterprise grade framework for business apps and not "hip" cutting-edge latest Chrome only thing. If I look around and see this blazing speed stuff changes in certain frameworks and what is cool and what is not I am really quite happy to have something reliable in terms of language.