Though I have issues with the Pokemon games on Switch, I believe there's no denying that the sales of the series have grown tremendously on Switch (as has nearly every Nintendo franchise that is on Switch compared to their predecessors).
I think DLC has helped actually, and your sales comparison at the end kind of proves that. SWSH managed to outsell Gens 3-7 even when including third versions (and SV likely will as well). On top of that, the Switch games cost more and we don't know how much of those 20m+ users purchased the DLC. Meanwhile with third versions, not everyone would double dip. Like if you had Diamond or Pearl, you might have thought Platinum was pointless (or you'd skip DP and wait for Pt).
In the Switch era, you'd get SWSH or SV and then keep all your data to be able to play more content through DLC.
