But that's just it. Given everything we've heard, the odds of backwards compatibility happening (at least for the console side of things) are quite slim. The PowerPC architecture is pretty much on its way out, which leave either ARM or x86 (& recent rumors indicate that the former will be used for the CPU).
I'd expect Smash 5 to happen around 2021 or so. As for a straight port followed by a sequel, isn't that what's rumored to be happening with Splatoon?
Here's the thing. You're hedging this on a rumour. And even then:
1 - The Smash Bros NX game is more or less a certainty. Two very strong sources has said it's true, one of which has said that numerous sources have guaranteed it.
2 - The Splatoon thing is a rumour that was noted to be very shakey, less certain than the SSB4 port, and having production problems. It may not end up even happening. Also note this was during the time NX was thought to be coming in 2016. If NX is 2017, that gives more time for a Splatoon sequel, so therefore it would be better to not bother with the port and just do a sequel.
Don't you think 4+ years is a bit long to wait for a new Smash Bros game, when it has historically been a system seller that makes a console more appealing to buyers? Given how hard the developers worked on SSB4 up until just a few months ago, how much content are you expecting the new game to have if they're tossing 4 years of work already done out the window? How do you think fans and the press would react to finding out that for the first four years of the system's life, the only Smash Bros game available is one they already played? Will NX even exist in the same form in these four years? Don't you think it's best to create an appealing entry in an established series as close to launch as possible?
Compare it to Zelda. The Wii U is criticized for not having a Zelda game until it's 4 years old, with the only main games until then being ports. They sold well, but not gangbusters, and the situation as a whole has left a dire taste in some people's mouths. Imagine how people would feel if they perceived Nintendo was allowing this to happen to more of their premier series. It's not a good look.
In general, this is the key question.
What would be the benefit of a straight port instead of an enhanced port other than you and the small niche that is the tournament community don't personally like SSB4?
I'm keeping my expectations low personally. It very well could be a straight port. But I don't think that would be a smart decision for Nintendo to do when new content and characters are the primary driver of excitement for the franchise and its creator.