They're screwed. There is no good way to release this game.
Each customer has a minimum set of things that must be supported or the game will be considered incomplete. This set will vary from customer to customer, but WotC can no doubt aggregate that to put together a set of stuff 'required' by the bulk of their customers - and they'll want to get that into print as soon as possible.
However, each customer also has a point where they decide they have enough stuff - provided these items are there, I don't need anything else. And beyond that they might buy more, or not. Of course, that point varies from customer to customer, but the percentage inevitably increases as more stuff is published, and so sales of books decrease with time. Eventually, they drop to a point where it's not worth publishing any more, and then it's time for a new edition. WotC, of course, want to delay this as long as possible.
The problem is that the 'required' stuff is almost certainly considered 'enough' by a large segment of their fans. (Indeed, WotC have noted this before - the majority of players buy nothing, a very large chunk buy the PHB and nothing else, a smaller number buy a PHB and a splatbook for their class or race, and after that the numbers drop dramatically.)
It may very well be the case that 5e is considered 'incomplete' right up to the point where it supports a dozen or so races, a score or so classes, across the full level range, and with 500 or so monsters... and as soon as it does support all those things, it immediately reaches the point where it is no longer worth publishing anything else.
In theory, the way around this is to focus instead on selling 'disposable' items - publish a setting with a metaplot that regularly updates, sell adventures, perhaps sell power cards or Fortune Cards, or... but I doubt those would sell enough to sustain the line - most people seem to go to WotC for rules.
The one saving grace they have is the DDI. This may well be enough by itself to keep the game going, and in time replace the printed line of supplements. But even here there's a problem - if most of your customers have enough and you're just drip-feeding new material to your ~80k subscribers via eDragon, you don't really need a permanent staff of designers and developers; you need one or two guys to coordinate the new material and run the e-magazines.