I tried Discord one time. I was quite confused at first. I still am. My initial impressions were that it was some sort of "glorified chat room/IM service". I used it to ask questions for one of the Final Fantasy game remakes. As mentioned by a few posts here, it didn't seem to be the appropriate utility/software for question based topics, so that did get me to write it off, and just stick with message forums.
However, I have been made aware, even back then, that Discord has gone up in popularity. So for naysayers (including myself :\), it's here to stay, and worth considering as a viable means to discuss one's topics of interest).
To get back to the original question, I think an aspect of the change in Dominion's popularity has to be that it basically birthed a huge genre which is the deck-building game. Regardless of whether you consider other deck-builders to be Dominion's equal, the fact is that while people who like deck-building games used to have one place to go and it was Dominion, now they have 1,000 places to go and only some of them are Dominion.
But that probably goes both ways -- all those other deck-building games are entry points that will lead some percentage of people who discover them back to Dominion.
I've been going to board game conventions for the better part of the past decade. I remember my first game Dominion in 2008...
I was at a con that focuses on eurogames (although other types/genres were welcome). Trying out Dominion was on my "to do" list, but I was playing so many other games that by the time I got around to this, it was already 2am. I looked around, there were still 10+ tables of gaming going on. One game I couldn't get in.. they were full. Another one, I did get in. 1 guy could teach, and the other 2 players were new. We ended up playing 2 games (The First Game and Big Money presets). It was awesome
Over the next few years, I'd see quite a few games of Dom. being played as a wandered the floors of such conventions. They also got played at various, meet ups, get togethers of gaming groups. Intrigue, Seaside, Alchemy, and Prosperity.. those were hot expansions!
In the past few years, Dominion either is nowhere to be seen, or perhaps you may see a game or 2 of it in play. But that's out of "a sea of tables" (up to 40 tables in some cases). Or, I see someone with a very large box, I look inside, and see he's got 10+ sets of Dominion cards in there, so even though that copy wasn't being played at the moment I walked by, it does show someone has it and brought it in.
Dominion is still great, but there are just soo many other great games out there, let alone great DBG. Kickstarter exploding in popularity helped get even more games out onto the market otherwise.