Thats why in the real world BA’s right business requirements which are signed off by stakeholders, which also include UX design which is a function not a job title. Validation as you put it, is part of the BRD in waterfall/rad, the SAD aligns with the BRD so the business required validation aligns with the system designed validation and when system testing occurs these validations are carried over without being lost.
I like to run fast iterations, I see why people think waterfall cant be fast or on the fly, that the project sponsor is getting chunks of development up front in their face weekly. whilst having the rigidity of the waterfall SDLC.
Ok some people want this UX Designer thing to be a seperate function, I dont know why, the more you know the more valuable you are. BA’s are the designers and one who cant do UX isnt worth hiring, that simple. Like wise call the BA a UX designer, if they dont know SDLC of whatever methodology you want from start to fisnish then they arent worth hiring.
Next Program Managers will be called “Gant Chart Checkers” because thats one of their functions so lets break down Progam management and over complicate SDLC for no good reason. Hybrid RAD and Waterfall has been working for years, its just the Agile guys hadnt worked in jig saw iterative based hybrid SDLC with customers before. So it needed somebody to pretend it was new and give it all a new name. My bad, I apologise.
I do hope one day though the Soft Dev industry will stop inventing red tape for the sake of it and just use existing methodologies that are flexible, iterative and fast, if you want them to be.
Just some banter and my 2c.