How popular would a Mario Maker esque Umihara Kawase Maker be?
Well let's see: it would be an entirely user-generated game based on a series with a miniscule fan base, not to mention published by a publisher who probably isn't doing to well, given that they've chosen to publish such a game. Content would be sparse, sales would be low, and servers would be shut down, leaving would-be buyers with a dead-weight copy on their hands.
Releasing the Trilogy along with it and having an online level-creating mode would be a better choice. Let's face it- no matter how much we prefer Umihara to Mario, Umihara isn't Mario. Umihara can't rely on huge, multinational fanbases of customers who know the games and mechanics in and out. Mario Maker can get away with not including the original games because they're already available in a number of different places, such as the Wii U's VC. If I want to play the Umihara games for reference, where can I go besides the Vita, PC, or old import consoles? They're not as widespread as the Mario games are. Plus, including three games and an editor are two selling points that would rub off on each other and make the package more appealing. New players could get cracking on the time-tested trilogy while veterans could put their knowledge into creating interesting levels. New players could start with Sayonara, then go to Original, then go to Shun, and finally try out the no doubt romhacky user-generated levels to put their old hand skills to the test. It would be an interesting tier system that keeps players going. The whole package, when included with a Trilogy release (on the PS4) would be well worth $40 as well as a Playstation Plus subscription. And even if you're an outsider and one of those four parts doesn't appeal to you, it would still broaden the series' appeal to the aforementioned outsiders a bit and give the veterans who have had the same three games for over two decades something new, with little extra effort on the part of the developers.