Yes, it should have been, but that doesn't mean it could have been. Usually, games that allow straight-up DLC had the intention of being upgraded/updated like that from the beginning. USUM was only thought of after SM had been already finished (or at least finalized in terms of features). Therefore, it makes me wonder if SM was capable of being upgraded with DLC in the first place. If SM couldn't be upgraded like that, then the alternatives would have been to either have no more 3DS games after SM and make the wait for Switch longer than it should be, or add more changes to USUM that they weren't able to put in due to time constraints and being pushed by Nintendo to focus more on the Switch game than anything else.
If you prefer the former, then I need to ask you this: does SM have enough content to keep us tied over? I think the Battle Royal Dome has plenty of potential to hold us over, but I'm in the minority when it comes to that. Especially since they admitted the Trails were rushed due to half of the development team being pressured to move onto the Switch, it's clear that SM has had trouble in its development. Put if you put together the rushed development, the fact that Gen 7 would have only one game that wasn't compatible with DLC, and that the Battle Royal Dome doesn't seem to be enough for people in terms of post-game content, I can see why Game Freak thought USUM was necessary. It doesn't seem like they succeeded though judging from everyone's disappointment.