By that reasoning, Dreamcast was a 5th gen system against the N64 and PS1, as for most of its life, it was up against those platforms.
1. I've explained the DC's situation before in other posts. To reiterate, first off, it launched closer to the PS2 than to the PS1 & N64. Also, it was discontinued prematurely. Had it lived a more normal lifespan, it would have spent the majority of its years competing directly with the other sixth-gen systems. Ergo, the Dreamcast is a sixth-gen system.
I really don't see why the length of the Wii/PS3/360 primacy should mean the 9th gen is going to be delayed further. On the other hand, the pattern of generations starting around the time that the generation two prior to it ended has endured through everything... so why should it change now?
2. Because the pattern was coincidental. In Japan, the only time the first system in a generation was released the same year as the last system from two gens prior was discontinued was in 2005, when the PS1 was discontinued globally in March and the 360 was annouced in May and released that November. In Japan, the NES & SNES weren't discontinued until 2003. Also, release dates are rarely global for systems, and since new systems used to come out earlier in Japan the generations started earlier there. If you want, I could make a Japanese counterpart to my previous timeline chart, and this time I'd go through the effort of trying to put the points at about the proper date instead of just on the line of the release year (my last chart was a quick & lazy one I churned out in a few minutes).
Incidentally, there's really no reason to think that the XBT and PS5 will be released as late as 2020, right now. It's based on the assumption that this generation will go somewhere near as long as the last one did, despite the last generation being a far stronger generation in terms of sales of all platforms. The general pattern is a 5 year gap between generations. This would see the next systems coming out in 2018, which is right about where we'd expect it to happen, if the only thing that happens with the rumoured "upgrades" is a resolution increase.
3. It's going to depend on the overall sales trajectory of the PS4 & XBO. Typically, a system isn't replaced until at least a couple of years after it peaks and sales have declined enough to warrant replacement. The PS4 & XBO may not peak until next year. This means we won't see them replaced in 2018 at that's too early. If 2018 as a whole is down enough YoY from 2017, we could see a 2019 release for their successors. If they decline a bit more slowly, we could see a 2020 release. Had the XBO & PS4 peaked last year or this year, then a 2018 release might have been more likely.
And if I'm right, and the NX (console) doesn't release until 2017, then there'll be a 1 year gap. But even if it's a 3 year gap, that really doesn't mean much on its own. The Wii U was in the same generation as the XBO and PS4. The NX is the successor of the Wii U. Therefore, the NX is 9th gen. If both Sony and MS wait until around the time that NX is being replaced by its successor to release their successors, then they will have kept the same systems for both the 8th and 9th generation of consoles. Because that's how generations work.
4. And once again, it's not as simple as "NES=3rd gen, ... Wii U = 8th gen, NX = 9th gen." There is at least one example of a company releasing two distinct platforms in a single generation. The NX's classification should depend on what it will be competing against mostly. If it spends its prime years competing with the PS4 & XBO, sharing most of the same games, etc., then it should be classified as 8th-gen.
1. That sounds like a completely arbitrary assertion. The Dreamcast spent the majority of its life competing against 5th gen systems. The fact that it was EXPECTED to last longer is irrelevant to that point. So if you define a system by which consoles it spent most of its time competing against, it's 5th gen. If you then invoke which systems it is meant to compete against, then the NX is releasing quite early, but is still 9th gen, because Nintendo isn't releasing it to compete against PS4 and XBO. It's releasing it as the next generation.
2. The pattern isn't coincidental. It just doesn't apply in all regions. With certain special exceptions such as Japan, system makers don't want to have three distinct generations of systems competing at the same time. So they discontinue the system two generations back, if it's still alive. If not for the newest generation, the longest-lasting of the systems from that generation two back would just keep selling as a budget system. And the fact that the exact discontinuation might not align perfectly within the year doesn't really matter - it's a ballpark indication, anyway. PS2 was discontinued in early 2013 instead of late 2012... which is still the same financial year, and might as well be the same year given that you're talking a matter of just a few months of overlap.
3. Systems get replaced when the maker decides that there is benefit to releasing a new system, and the system is ready. Usually, that means about 5 years after the previous generation, when power improvements are strong enough. As mentioned by HoloDust, there was a delay in improvements during the 7th generation. That meant that console makers weren't ready to move to the next generation, yet. And as a result, software support didn't decline due to teams being shifted to the new system, and sales didn't decrease. In short, you confuse cause and effect. Sales decrease because of the output decreasing, which is caused by the maker shifting resources. The new system isn't released because of the sales decrease, the sales decrease occurs due to the planned new system. Note, by the way, that Wii U sales have been pretty flat over time - there has been no "peak" for the Wii U. The Wii U sold roughly as well in 2015 as it did in 2014, and 2014 was a record year for Wii U sales. And sales of Wii U in the last three months has been roughly equal to that of the first three months of 2015.
4. You're not referring to Atari 5200, are you? Because the first two gens were prior to the generation cycle we now find ourselves in. If it weren't for the crash, it would be considered the start of the 3rd gen, alongside Colecovision. And yes, it actually works out to be exactly as simple as "3rd gen = NES, etc". Every single generation since the crash has seen this exact pattern. And the Atari 5200 would be Atari's 2nd generation of console - as I said, the only reason it's not considered 3rd gen is that the crash wiped out the entire generation, and we ignore it. Hence the 2 year gap between the 2nd and 3rd gens. If not for the crash, we'd be calling the current generation the 9th generation, and debating whether NX was 10th generation. And the 2nd generation would last until 1985, and the 3rd generation would last until 1989, and then the 4th generation would start in 1985 with the NES.