There are certainly some issues with the genre itself, but that's greatly exacerbated by technical issues, content issues, bad marketing, terrible release dates, etc... So let's take some of the OP's examples one by one:
- Deus Ex Mankind Divided: The game had technical issues at release, marketing around pre-orders had all kinds of shenanigans, microtransactions were shoved into SP game at the last mode, some random MP mode was added, game felt "short", story was cut in half it seems, the whole racism against cyborgs shtick was not well done, development took a long time, critical receptions suffered and game sales suffered. So it wasn't just the genre that was the culprit here. Deus Ex HR was a much better overall package, and Deus Ex MD seemed more of the same but perhaps worse and with less content.
- Dishonored 2: The game seemed pretty good but it had terrible technical issues on PC, and some on consoles, marketing seemed quite a bit off, and furthermore it had a horrible, horrible release date. I mean, seriously, who the hell decided on that one!
- Prey: The most niche of the 3 games. Marketing seemed very shallow. The game once again had some technical issues around it. I just don't know if this sort of the game really sells well in today's market (unlike Deux Ex and Dishonored games which I feel have more potential).
Some of the other notable failures mentioned:
- Watchdogs 2: After Watchdogs 1 that had to be expected. The first game was a massive disappointment to quite a few people so naturally the sequel wasn't going to do so well.
- Titanfall 2: MP completely changed its "flow" from Titanfall 1 which was where the game shined. SP seemed pretty good if short, but considering changes to MP, that wasn't quite enough. And finally, the most terrible idea was to sandwich release of this between Battlefield and COD. That was truly a genius sort of a decision.
- SFV: Yeah... Don't want to even bother with responding on this fiasco.
- ME:A : Everyone knows what happened with this one.
So, in a majority of cases, it wasn't the genre (or even sometimes the game) that was the culprit. There are a LOT of other factors. On the other hand, lets take a look at successes:
Nier, Nioh, P5, Horizon, Ratchet and Clank, Crash Bandicoot all sold pretty well. And before that we had quite large sames for Fallout 4, Witcher 3, Doom, etc...