Phil won't be doing this for 20 years. The biggest reason IMO for his huge losses in views and fabs is simply that times change and he hasn't. Of course there's a bunch of other factors like his attitude towards his fans and his reputation but I think the biggest one is this: He's still acting like it's 2009.
We know that Phil's majority audience is kids. Its always has been. Always will be. There's actually nothing wrong with that, as he's shown, they're the most passionate audience that you can have. The reason why he was so big in 2009 - 2012 was because his childish humour and jokes fit the youth of 2009 - 2012 so he was seen as cool. Any jokes he made were relevant at the time and he made them laugh, he entertained them. Now while young audiences may be beneficial in that they're passionate, dedicated and don't really know better so you can sell all sorts of crap to them, the drawback is those young audiences grow up, no longer find that stuff funny and inevitably move on as they find other interests and their life gets busier. This is why kids TV shows tend to only last 4 - 6 seasons, as the next generation of kids will more than likely have no interest in the show. Or if its a long running show they'll do stuff like completely change up the cast and the whole style of the show to keep it fresh, or something like Power Rangers that completely changes from year to year.
Its the same thing with Youtube shit. What AVGN did to keep himself relevant is essentially grow up with his audience so he doesn't lose them. He no longer has the same toilet humour he did in 2007, and more often now he does stuff as James Rolfe than as AVGN, which helped him stick around for this long of a time without a massive popularity dump, as he got rid of the gag that was getting old and instead grew up with his audience. Rooster Teeth, they constantly changed what the hell they are for a while, lets plays, reviewers, gameplay tips, all that stuff, now they've grown enough to do all at once and more including animations and tabletop stuff, now they grab all kinds of audiences. Even Pewdiepie and Jackscepticeye, they no longer do the same shit they used to because they gotta stay relevant. The stuff they did with screaming at horror games is old, thats 2012 stuff. 12 year olds in 2012 are different to 12 year olds in 2018. So they moved on. DSP is STILL in 2009, nevermind 2012. In fact any major change in audience interests has caused him to say its a "fad" and meant he's never took part in it because he loves to be Mr. Contrarian who goes against the system. They're not fads, they're a new set of kids who are interested in different things and that "fad" is their interest that they grow out of as they get older, its not the same set of people moving from "fad" to "fad" they're all completely different kids just playing and watching whats popular at the time.
So whats been going on with DSP is he loses more kids each time as they grow older and either grow out of his humour and don't find him funny because they find the humour immature, or start to see his flaws as they're older and wiser and start to see evidence. Any diehards that grow old and remain behind inevitably leave as he drives them away with his behaviour. So as he bleeds an audience as more kids grow older, he doesn't gain enough kids to replace the audience he lost, which is what other online entertainers aimed at kids do, they lose an audience as that audience grows up and then that audience is simply replaced by the next generation of kids as the entertainer adapts to whats cool for those kids. DSP doesn't do that, he still does shit from 2009. So as his audience grows up and moves on, he gets like half the audience he lost in new kids as those set of kids don't find him funny or cool because he doesn't click with them. He's dated, he does jokes and references from the 2000s, he doesn't play whats cool these days, and when he does, its always way too late when its old news. DSP likes to say he's changed a ton because he livestreams and direct captures and all that aesthetic stuff, but his commentary and the overall meat of his product is now 10 years old and he grows more and more distant to what the youth, which tends to be the biggest chunk of his audience, wants to see today.
Playing the hottest new AAA console release was all fine and dandy in 2009 but in 2018 a kid will go "Who plays console these days? I'm going to go and play Fortnite"
"I used to be good at this." 
