But that's not a limitation of RAD. Delphi 4 and Visual Basic both had "align and/or resize proportionally" on all axis.
Making an UI that adapted in size from 640 to 1280 was a 4 clicks matter.
Having COMPLETLY SEPARATE UI for different form factor like we need on the web sure, but that shouldn't be an issue on mobile just pack X version of your UI like you end up doing with all your resources anyway.
> RAD, a.k.a. UI-nailed-down-to-pixel-grid approach
That's really not what RAD was about.
RAD was about "every single component you use is generic, you don't care how it works, you only want to say where it is, what event you listen to, and optionally do some custom rendering".
As said above, positionning could be non-fixed, auto-adapted to the view, ...
RAD, a.k.a. UI-nailed-down-to-pixel-grid approach, has died immediately after we've switched from 640x480 pixel screens to variety of display and devices.
If in doubts think about Adaptive and Responsive Designs ( https://en.wikipedia.org/wiki/Responsive_web_design ) in regards of RAD.