They now even have slides inside slides, so we can conclude there is much demand for them.
No doubt they add floor space, but it would be easier to make an RV longer if square footage was the only measure. What slides mostly do is add width which then allows designers to create different floorplans. However, the wider the slides, or the narrower the RV, the more they crowd the interior when retracted.
Over the years I've looked at 100s if not 1000s of RVs at shows, and they are essentially never displayed with slides in. I'd bet that most buyers would pass on many RVs if they walked inside with slides in. They obscure visibility while traveling, diminish access to bathrooms and kitchens, reduce inside and outside storage, increase chance of leaks, make floors uneven, add weight, add cost, add maintenance, etc... I could go on but for me the chance of water leaks is enough. The musty smell of RVs only a few years old literally makes me sick, and it's far too common with slides. Shouldn't be that way but it is. Apparently too many owners don't follow instructions on how to prevent leaks according to manufacturers.
Vans are unique in that they have rear doors to improve flexibility. They can be opened for loading, ventilation, emergency exit, etc... To eliminate that feature seems questionable to me. Some vans have slides behind driver that extend out through optional side door, and that is slightly more appealing to me. But not by much.
The Roadtrek in OP would work just as well for me if it had a dinette in rear that made into a bed. And I'd like access through the rear doors more. The seat looks cramped anyway.