Spolier: Do not put 'listening to leftists' as your answer, too easy.
With the paracite hangers on of game 'journalists' the first to go, the amount of bleakness from Ubisoft, EA trying to sell itself harder than a crackwhore, we are approaching a point where we may end up with only 3-5 major ones left and a few focused on one title that's been around for decades (eve online, your DOTA games etc).
Doesn't really affect us as I think I'm safe in saying we all have backlogs that can outlast us being on Earth! So why not make this a fun opportunity to go 'you know, if you did this , you wouldn't be begging for money'. Choose whichever companies you want on what you think was a missed opportunity:
EA: Not using their IPs together enough. Seriously how hard would it have been to make a Battlefield game set in the Command and Conquer Red Alert or Tiberium franchise or even do a sports game but in the backdrop of Mass effect. They seemed to be all about processing IPs than using them effectively.
Bethesda: not using an Id engine for Fallout 4. When I played Rage, the story was lackluster but the combat and gameplay was smooth, fun with fantastic visuals. Unless there was a hidden licensing reason, why not use THAT for Fallout 4, that seemed like the perfect engine .
Nintendo: making it too difficult to buy and play older titles so creating a piracy market.
Microsoft: not letting gamers be the one to design their military AR project as they could've probably told you the dumb parts quickly.
Sony: Not embracing Japanese game devs, not leveraging their monopoly on anime with PS+ by doing, for example, free first 3 episodes of a series on Crunchyroll if you have PS+. Also on Crunchyroll, just put an adults only section for uncensored anime, have it behind a paywall of some small fee and a SECOND password and you'll probably get a lot more subscribers.
There's more I could mention like Ubisoft, Rockstar, CD projekt etc but gone on too long and let this be a collective 'I would've done this' thread.
Counterpoint: the only gaming market collapsing is AAA. Indi devs will inevitably rise to take their place
Over corporatized business practices is what did it. Big expensive meticulously planned cgi slugs are all they can do. Nintendo as well don't let the cute graphics fool you. And because they are so big and expensive and time consuming-- many new games take years to build-- they have to be runaway hits otherwise it's a massive loss studio's cannot survive
They simply cannot afford a misstep. And because of that they stagnate and collapse. Too afraid to fail to be unique and innovative and potentially plunder much bigger riches therin. (Potentially - being the scariest word there is)