The small amount of new stages I don't think has anything to do with the amount of veteran characters. If anything, it's more likely it's due to the team bringing back 90+ old stages which obviously required a ton of work since aside from the Smash Wii U stages they all come from the pre-HD era.
Even the Melee and Brawl stages that were featured in Wii U got a dramatic increase in graphical quality in Ultimate. Just look at Halberd or Port Town, in Smash U they're just ported from Brawl while in Ultimate they're fully retextured and re-lightened.
Well, if we're going to go there, I also would've been more than fine with fewer older stages in exchange for more new stages. Especially if you're agreeing that the old stages took a ton of work, mitigating the possible downside of newer stages potentially being harder to create. I also would've been fine with cutting some veterans in exchange for those new stages if necessary, perhaps a more controversial take than the former.
But anyway, the question is whether resources freed up by cutting vets would've/could've been directed towards stages. Well, I think it's easily possible, and nothing is truly independent of anything else given that they're working under budget and time constraints. But, regardless, the overall point is that cutting some vets frees up a ton of resources, and that manpower and budget could've easily gone into something else, whether that be characters, stages, or besides.
And yeah of course developing the veterans also takes time, but this boils down to roster complaints. They didn't "sacrifice" anything. They opted to include X and Y characters instead of A and B. Most of those X and Y characters happened to be veterans. That's it.
How much you appreciate the choice is up to personal taste.
Yes, they chose to include more veterans instead of more new stuff. That's exactly what I've been saying, and exactly what I mean when I say they made sacrifices in other areas to do EiH. There are upsides and downsides to each option, and EiH isn't automatically superior in all facets.
Shulk and Bowser Jr. for my personal taste are two of the most zzzzzz characters in series history, I would have taken Lucas and Roy in the base game over them any day of the week.
Well, that's up to you as to who you'd personally prefer. However, by any objective measure I think Shulk and Jr., a highly unique brand new franchise rep and an incredibly popular Mario staple, win out handily in overall appeal and game/roster quality over two JP-exclusive clones...but that's besides the point, because it would never be that two-for-two swap anyway. The point is only that you'd have to make major concessions with the newcomer roster and beyond in order to achieve EiH in the Smash 4 scenario, and I really can't fathom that doing so would've resulted in a better overall product in that case.