Here's a thought.
If I were WotC I would consider releasing a rules update book every year or perhaps every 2 years. Now before people jump all over me let me explain. First, a rules update and not an edition change would allow people who are interested in updating to update and those who don't see the need to simply avoid buying it. It allows the game to become a living, evolving rules set. By no means am I advocating massive rules changes but large enough changes to warrant a book. For example, an overhaul of the metamagic system, spell rewrite (AKA polymorph as a past example) and for clarifying existing rules are examples that come to mind.
From WotC point of view it would accomplish 2 things. 1) A book that most players/DM's would buy and thus would give them 1 really good selling book a year on par or close to in sales comparison to the PHB, DMG and the MM which is something most other products of theirs does not meet. 2) New products could then be released based on the new rules updates. This ensures a continuing source of new and hopefully interesting products to accompany the new rules which in turn helps to create a stronger market for purchasing new books. Which of course then create a profitable company and reduces the need for massive revision changes (4e, 5e, etc).
From a customers perspective this will have a limited impact on their play. For those customers who buy most of the books, buying these books would not be an issue. It will simply become part of their libraries. For those out there who buy very few books (and this includes some of my playing group), 1 book a year isn't going to break you, especially if it has some crunch to it that helps to improve the quality of gameplay or enhance/balance the game. Really it comes down to, if you want it buy it, if you don't then don't buy it. However, I suspect a majority of customers would buy it.
Although this is not perfect, it does seem to strike a balance from the WotC perspective and the customers perspective. I guess the main drawback I can see is that people may be playing with different rules sets from group to group, but in reality that is beginning to happen more and more with each edition released. There are some who only play Basic still, others 1e, others 2e, others 3e and still others on 3.5e. So as time grows and more editions are released it is only going to get more and mroe fractured. In addition, those players who attend Gen Con and that sort of thing tend to be up on the latest rules (I could be wrong here) and so most players attending gaming conventions will likely be using are at the very least be familiar with the latest rules. For those groups who do not attend gaming conventions then really what does it matter what rules subset they are on.
Food for thought.
PS - One more thing. If WotC were to consider something like this they should inform their customers that this is their approach to the game from this point on (that is at the time of adopting rules updates like this one). It would lessen confusion and prepare the mental state of the their customers that this is how their game will be supported for the foreseeable future. Then those who would be on board with this will have an understanding of how the game will evolve and for god's sake it will kill all the 4e, 5e edition threads that pop up all the time.
