I want to assure you that UI for ASP.NET AJAX is alive and kicking. For example, the R2 2018 release that will be live by the end of the week will contain an upgrade to jQuery 3.3.1.
We also apply critical fixes like those caused by browser changes (for example, Chrome recently changed scrolling containers and basically broke all popups positions, Edge 41 broke rich text inputs, etc.) and we do take care of those in order to keep the suite up to date and working properly in the current browsers.
That said, new development is, indeed, happening at a slower pace because demand for it is much lower than it used to be. I agree completely with you that overhauling the frozen columns and/or frozen headers functionalities would be grand (a lot of that was written in the IE6-7 era when modern approaches like those used in Kendo were just wishful thinking), but this is a humongous effort, because they contain a lot of functionality and we must be extremely wary of regression issues. At the moment stability is what is most required of the suite, so such overhauls will not be welcomed by most people as they will also bring breaking changes to custom code (like DOM traversals). From a business perspective, it also makes more sense to invest that time in tasks where it will have a bigger impact on more customers because this will allow us to improve the life of many more developers. For example, creating a native set of React components.
I hope that such a short and candid answer helps. Let me know if you have any outstanding questions on this matter.