Considering they're 2 versions of the same game, combined they're already the best selling Smash Bros, which means Nintendo can not get away with just releasing an enhanced port.
I disagree. Port or not, the series is bankable. A port would sell well; a brand new game may sell more. Either way, it's a win for Nintendo. People who post on video game message boards may be privy to the development history of the 3DS and Wii U games. I wonder how many people view them as "two versions of the same game." While they share content, the 3DS version is a severely stripped down experience missing features from previous installments played using a Circle Pad. Still pretty good, but let's call it how it is.
The Wii U game, the one used in tournaments with an audience Nintendo is increasingly paying more attention to, flat out performed worse. Nintendo spent significantly more resources on the version that sold significantly fewer copies. I'd imagine Nintendo wants more mileage out of that investment.
Plus I don't see why some think they wouldn't be able to get a new Smash Bros this year. They made Smash Bros 3DS/Wii U in a little over 2 years since development started during Summer 2012 with that game, and since they wouldn't have to worry about making a 3DS version as well this time, they would have had more then enough time to create enough new content.
There's a lot to unwrap here.
It's physically possible for Sakurai and [insert developer here] to develop a new Super Smash Bros. in the time between when Bayonetta DLC released and the end of 2018 (as the rumor suggests). However, it doesn't seem particularly likely.
1. Sakurai left HAL Labs because he was tired of the expectation that sequels were inevitable and had to be asked to return to direct the next two installments (as an independent contractor) by Iwata who had the benefit of decades of friendship. For a brand new Super Smash Bros installment, we would have to assume that Sakurai, after completing two games he slaved over for three years to the detriment of his own health, was eager to jump back to work on a sequel at best only a few months later.
2. Aside from key people like Sakurai, it’s doubtful that the 3DS and Wii U versions were developed by the same people. Since the games were developed concurrently rather than one after the other, not creating a separate version doesn't reduce development time. There are no real shortcuts to the work that is required to make something new and more importantly, good. Creating one version only means fewer people would be hired/contracted. Budgetary restraints mean a company can’t hire/contract indefinitely for the purpose of significantly speeding up development time; it hires/contracts many people as it thinks it needs. Once the project ends, contractors are let go/not renewed which leads into the last point.
3. For a project as large as a new Super Smash Bros., someone would have noticed an influx of hiring/contracting regardless of who the developer is. With all the leaks from Wii U/3DS, I'm not convinced Nintendo could have kept this quiet for so long. A port, even with additional content, doesn't require nearly as many people.
To be clear, I'm not saying a brand new game can't happen by the end of 2018, but there's enough stacked against it. Personally, Nintendo has me by the wallet when it comes to Super Smash Bros. I'll buy anything so my opinion on the matter has no bearing on whether I want a port/new game more.