That was a rare exception. FFVII did indeed open the floodgates for the PS1 in the U.S., and was arguably as important for the PS1 as SMB was for the NES, Sonic for the Genesis, and Halo for Xbox. There are system-sellers, then there are brand-makers, games so important they're arguably responsible for making a particular console brand a commercial success and a household name. Since there are no monthly U.S. sales figures for the Genesis and thus I can't say for sure about the effects of Sonic on Genesis hardware, FFVII is the only known game released after launch to cause a system with weak or otherwise unimpressive baseline sales (the PS1 was trailing the N64 during the Jan.-Aug. 1997 period in the U.S.) to suddenly reverse course and have very strong sales going forward, and even then that was mostly in the U.S. Its effect appears to have been smaller in Japan (where the PS1 was already starting to build up a significant audience in 1996). In Europe the PS1 appears to have already started to improve before FFVII, as shipments started to rapidly increase well before FFVII released in the region, though without actual sales data (because shipments ≠ sales and can only give us an upper limit on LTD sales) we can't say for sure.

If you'll excuse a rough estimate for the PS4's final U.S. & Europe tallies for 2017 (the actual final bar shouldn't be more than a couple of pixels off, though):

Note that Sony stopped breaking down PS2 shipments by region after Q1 2007.

Regarding system-sellers, lots of games can be system-sellers in the broadest possible sense that someone bought the system for that game. I bought an XBO for the MCC, for example, but that doesn't mean enough people did so to cause a measurable effect on sales. Likewise, I imagine there's probably more than a few people out there that might have bought a particular console for an even more niche title than a Halo compilation. But there are system-sellers and then there are system-sellers. I'm talking the ones that have a real impact, meaning significant month-over-month and/or year-over-year boosts, not the ones that pushed maybe a few hundred or a few thousand extra units. Those are hard to come by.

For the U.S., it's harder to tell because sales are tracked monthly by the sole "official" sales tracker and thus a bump that would be noticed in a single week would be "washed out" by monthly tracking (a 50% boost for one week would produce a roughly 12.5% boost for a 4-week month, barely noticeable and indiscernible from statistical noise), but there appear to be very few system-sellers here. This generation, the only games that caused a clear, demonstrable increase to sales that can't be explained by other factors are Destiny and its expansion The Taken King, Arkham Knight (and its special edition Batman-themed console), Halo 5, Mario Kart 8, and possibly Uncharted 4 and Super Mario Odyssey. That's it. Nothing else has produced an effect that is noticeable in the sales data.

Japan tracks data weekly, so there are more obvious system-sellers, but most have a very modest effect, with only a handful producing a significant enough increase in sales to where you'd notice the effect if sales were tracked monthly. GTAV, MGS5, FFXV, and DQXI were by far the biggest system-sellers for the PS4 in Japan. There's a few others, but they're blips in comparison. Most weekly data points show variation that can be chalked up to statistical noise, as the sales curve always kind of "wiggles" around week to week even in the absence of anything that can effect sales.

So, even when we have sales being tracked weekly, we see very few games that produce a significant uptick in hardware sales, and from what I can piece together, the Japanese hardware markets seems more responsive to software overall than the U.S. market does.

Regarding the Wii, it had a sales curve in line with other Nintendo consoles, just proportionally much larger. Nintendo's home consoles have always peaked early, and ever since the N64 they've had terrible legs because they've had terrible long-term support. I've noticed that systems that get abandoned after their successor comes out tend to die quickly. There's nothing to generate interest anymore (different phenomenon from system-selling titles), and no confidence in the market. The parent company has moved on, as have third parties, assuming they gave the system any appreciable amount of support. The same was the case with the Wii, and will probably be the case with the Switch once its successor comes out.

As for the PS4, in the U.S. it simply doesn't have the market share the PS2 did. The overall market for "conventional" consoles has remained steady, but even with the XBO failing to replicate the success of the 360, Xbox is still strong enough to prevent PlayStation from dominating like it once did. And the Japanese home console market has definitely declined. The Switch being a hybrid system is probably why it's selling so well. It's like the Japanese market views it as a handheld for the most part, though maybe not as a true 3DS successor seeing as it didn't have any immediate effect on 3DS sales.

As for Europe, the PS4 is so far tracking about on par or slightly ahead of the PS2 in terms of LTD sales. The dip in shipments in 2004 is most likely due to the same Slim shortages that affected the U.S., an issue that resolved itself in 2005. That being said, the PS2 did have strong legs. While the LTD shipment count as of the end of 2006 was at just over 44M (meaning it had to sell a bit less than that unless it totally sold out), it continued to have strong legs, and was at 55.28M shipped by the end of 2012. That means the PS4 will have to sell another 15M for 2018+2019 combined to keep pace with the PS2. That means that both years would have to average almost as well as 2017. That's not going to happen. And it would have to sell at least another 11M in 2020 onward, which I doubt will happen either. I personally think the PS4 will likely end up at around 45M lifetime in Europe. Well short of the PS2, but far ahead of the PS1 and enough to make it the second best-selling home console ever in the region.

I think its gonna be roughly the same as 2017... but abit lower.
So instead of doing 21m like in 2017... it probably does 20m (if things go well).

Even if its peaked in 2017..... .selling 20m is something, that would put it around 94m by early 2019.

I totally agree with this.  2017 is the most reasonable year for PS4 to peak, but I also expect it to keep going strong.  The PS4 seems like a fairly predictable console to me.  It sells more each year until it peaks in 2017, then sales gradually decrease but stay strong fairly strong.  It's sales are not going to fall off a cliff (like the Wii's last few years), and it's not going to be the best selling console of all time either.  It's a good solid console, no more and no less. 

It will and is a lot more then just a " solid" console.

A lot more, but we shall see at the end of this year and see what people have to say when 2018 sold more then in other years.