Ultimate was built from the ground up in Unreal Engine.
It was not built from the ground up, nor on the unreal engine. Building a smash game from the ground up takes a lot of time, more than they had. The unreal engine is a common misconception, it was shown in the credits at the end of e3 for other games. Looking at the end of a smash direct will disprove your statements about Unreal Engine.
It is not fair of Nintendo to purely show Smash, but it isn't fair either to criticize them on their decision to do so. Super Smash Brothers was one of the top 5 best selling games on the Wii U, below games that came pretty much years before it (the exception being Mario Kart 8). It makes sense that they would milk smash. The Switch is one of the best selling console's Nintendo has ever produced, and there are people who haven't ever played Smash brothers before, and Nintendo is trying to build hype for this game.
While this is happening, we also have to remember that Nintendo is 100% doing more behind the scenes. With smash ultimate taking the spotlight, games being developed (like Bayonetta, Metroid, and Yoshi) are all getting closer and closer to completion. What wouldn't surprise me is if the first year E3 was insane to sell units, this years E3 was lackluster to sell as many smash copies as it can, and next years E3 will drop countless loads on us. A bukakke of games, if you will.
I am not necessarily defending Nintendo, because I sure as hell want to see more MP4 stuff, as Metroid Prime is my favorite game series ever. I also want to see if Yoshi will be any good, because I have always disliked Yoshi games. I want to see more actual games, like SMM2.
Just keep in mind that being patient is worth it. After all, we have monster hunter coming out this month (a week from today no?), Pokemon in November, Smash in December, and definitely a more promising lineup next year.