The amount of forum users is honestly overrated. The biggest forums like Resetera have like 50,000 registered users, and that isn't even active users, probably only about 30,000 of that are active regular posters. This forum is maybe a few thousand regular posters maybe.
It's not like Nintendo gave any information, they just announced something called the NX and that it would be a dedicated game hardware, they would not comment on what it was, the hybrid nature, which games, what the hardware capability was, I don't think Nintendo even confirmed Nvidia was the chip supplier until October 2016.
EDIT: Here's actually an article that explains why Nintendo had to announce NX at that time (March 2015) ... NX was announced at the same investors event where Nintendo announced they would be working with DeNA on cell phone games, they had to announce NX to reassure investors that they weren't leaving the gaming hardware business.
The announcement was made during a hastily convened press conference in Tokyo on Tuesday. The main purpose of the event was to explain Nintendo’s decision to enter the smartphone gaming market, via a partnership with smartphone games specialist DeNA.
However, Nintendo’s chief executive, Satoru Iwata, went on to announce the hardware platform in order to assuage fears that Nintendo may move away from manufacturing dedicated games machines.
You're not going to see 1+ year advance hardware announcements any more I don't think unless a system has really flopped because all you're doing by announcing a system that far in advance is undercutting sales and probably a holiday season of existing hardware and there's no point to doing that.
4-5 months in the modern world is more than enough time to announce an electronics product and have it out for release. Cell phones get announced and are available the same week these days, even the old Apple cycle of announcing an iPhone and having it available maybe 6 weeks later is too much lead time these days.
You're forgetting the lurkers. Forum users, reddit users, Twitter users. Gaming has a huge number of people who follow news and rumors in advance. The average gamer is not the target of the release window.
The nature of smartphone market is a bit different. Two, three and even four lines of smartphones coexist in the market for a couple of years. Most of smartphones are functionally the same, just with some hardware upgrades. Even software support and SO is the same for different models, hence you don't need to feel totally alienated to not have the newest version.
In console one line pretty much replaces the other. It's expected the next generation games to not be always playable on your current consoles. Acquiring hardware is a mostly planned purchase, you get a console and calculate how many years of support and games you have on it before replacing your machine. The announcement of PS5 (one year before its launch) haven't killed PS4 momentum in any sense, because people who were waiting for the sixth year to get their FIRST PS4 were never the target of a launch window for PS5. The pool of consumers is essentially different. Even if some people decided to not buy a PS4 it didn't hurt Sony in long term, a weaker quarter sales can be easily maked up by a very strong PS5 launch which was precisely what happened
For manufacturers is actually good business if people adopt their next hardware asap, because the purpose of a new hardware is to replace the old one