If there is a buyer new shares will be issued or temporarily shorted until new supply will hit the market. There are other sources of shares because we have brilliant financial engineers. Simply put supply is unlimited. No worries.
I don't think it's that simple. First of all, a lot of the top-quality mid-to-small-cap companies are getting bought out. There have been plenty of articles lately lamenting shrinkage in the number of public companies, which has been cut in half over the past 20 years. There are a number of factors behind this, but surely ZIRP is making it much worse, given how easy it is to just issue a bunch of junk bonds (at what would otherwise be T-bill yields) to raise cash for acquisitions.
There's also plenty of bankruptcies due to ZIRP. MF Global a few years ago, as well as the many energy companies more recently, are a direct effect of debt binging and then things not going quite as planned. I think 0% interest rates also have a psychological effect on even the most "sophisticated" borrowers -- it's easy to "forget" that you still have to repay all this "cheap" money.
Younger people are less likely to start companies today. Again, there have been many articles over the last few years on this -- for example,
http://blogs.wsj.com/experts/2015/0...-starting-businesses-and-why-thats-a-problem/
Another issue with share counts is that many of the biggest silicon valley companies tend to have share classes that give most, if not all, of the voting power within the company only to the one or a handful founding members. So what John Q Public ends up holding is inferior, and that may become a problem as large corporate governance issues emerge (e.g., very bad management decisions, or executives using the company resources to pursue personal political agendas that the other shareholders may not agree with).