Per Wikipedia, there were over 1800 automobile manufacturers in the U.S. from 1896-1930. Over a 34-year period, that averages out to almost 53 new car manufacturers a year - over one a week!
Needless to say, that's far too many, especially for a fledgling industry. Thus, it's not a surprise that few of them were able to endure the test of time.
In fact, a large number of them practically withered on the vine, lasting no more than a few years. (The Acme Motor Car Company, whose ad is shown here, actually lasted 8 years, from 1903-1911, which is surprisingly lengthy compared to many of its competitors.)
In a similar vein, there are some who say that the world of publishing is undergoing a comparable expansion. Basically, with anyone being able to publish whatever they like at the push of a button, we now have millions of books being produced outside of traditional channels. However, just like the auto industry couldn't support almost 2000 manufacturers, there are some who think that the current number of indie authors will eventually lead to a shakeout - a thinning of the herd - because it's simply not possible for every indie author to earn a living writing books.
Personally, I'm not convinced that there's an exact parallel. For starters, not everyone who publishes a book does it for profit or with the intent or hope of becoming a full-time writer. For some, it's simply an item on their bucket list, like climbing Mount Everest or going skydiving; for others, it's just that one story that they have to tell. (I suppose it's a lot like a person who works in their garden or goes to the gym every day - they do it because they enjoy it, not because they're looking to grow a prize-winning rosebush or be crowned Mr. Olympia.) Frankly speaking, these are people who may be one-and-done, but as soon as one leaves someone else with the same one-book desire steps in to replace them.
Of course, there are other people with a profit motive who will publish and ultimately abandon the idea of being an indie. A number of them will decide that it isn't profitable for them. Many may find that they hate the process or that it's too time-consuming. Still others will decide that they only want to be traditionally published. As before, though, as soon as someone in this category departs, there's another individual waiting in the wings to fill the void created by their exit.
Bearing all this in mind, it's easy to see how some might feel that there's a glut of books flooding the market. In retrospect, I suppose the ultimate question is: Can we have too many writers (or at least, too many indie writers)? While it's not on par with asking if we can have too many doctors or too many teachers, I certainly don't think so - especially not when everyone's motive for writing is so different. That said, I think there will be a shakeout in the sense that there are people who are publishing today who may not be doing so - for various reasons - two years down the road. However, self-publishing these days is a business with very low barriers to entry (for which I am grateful), so I believe that there won't be a significant net loss, if any, in terms of the overall number of people who consider themsleves indie authors. And that's a good thing, in my opinion.