I'll disagree with you. I/we buy 3rd party controls to speed up our development NOT to slow it down, we just what them to look reasonably modern and we do NOT want to get that involved in UI as small development groups rarely have resources or the budget for dedicated "UI developers" ... just not how the real world works. Small dev teams have a very different focus and hence their desire to purchase 3rd party controls that just get the job done with minimal effort. If we had a UI developer, then we probably wouldn't be using 3rd party controls at all.
But what if I don't want the change to impact my entire application? What if I just want a Red Tab, nothing more, nothing less? What if I don't use animations because they are resource intensive and provide NO benefit to my clients? I don't know who your majority clients are and I really should need to know ... again, 3rd party controls are about RAD, we buy controls so we DON'T have to focus on the UI.
Telerik's no-xaml/xaml binaries was a VERY bad decision for those of us that need to focus on other aspect of core development NOT the UI. We just don't need "themes" as it does nothing for our clients and doesn't help us generate revenue for my company ... they need speed, performance, and a "good enough" UI that looks reasonably modern.
I realize Silverlight has long ago been abandon by Microsoft and as such is probably not a focal point for Telerik, but your company still sells Silverlight UI controls and still updates them (and we've been long time subscribers). I feel your focus is simply wrong. Again, it's all about rapid application development (RAD), there is NOT going to be a long term re-usability with Silverlight (come 2021 it'll no longer be support by any browser). Code re-use at the UI level is close to non-existent, the re-use is primarily in business logic and back-end SQL/Database. UI's change so frequently it just doesn't make sense to chase that white rabbit from a development resource standpoint.
Sadly there is NO web technology that is as powerful as Silverlight ... it's ability to look like Windows forms apps (OOB) is the primary reason we're still developing with Silverlight - our clients demand a better UI experience that what HTML5 can offer ... enter Silverlight OOB. HTML5 can't operate anywhere near the performance and consistency of Silverlight ... in fact HTML5 is not even an option for my company going forward (restrictions on simple things like printing, setting control focus, and host of other restrictions that may or may not work pending browser used and sub versions ... chasing that is a fools game and a huge waste of development resources) . We're going to shift back to Windows Forms (maybe WPF if we can fine the right 3rd party controls that focus on RAD). Windows forms apps are just so much more secure than HTML5 and/or Silverlight.
But it sounds like what you are suggesting is that Telerik is not a good fit for RAD and my company's development needs.