If you really wanted to maximize how many people played your game, you would support 16-bit.
Well, if your game is really going to work in all other respects on a videocard which is 10-12 years old and probably hasn't had any new drivers released for five years, I guess.
I never really found it much more work to implement.
It's not about the amount of work, it's about making the experience worse for more users than you can improve it. For every one person who really can't use 32 bit colour who gets to play your game in 16 bit colour, ten other people suffer glitchy graphics because you didn't tell them to switch to 32 bit colour in the control panel. You see, you don't just get less colours in 16 bit video mode, you also get a vastly decreased zbuffer, which means far more vertex wobbling and depth sorting issues. Not to mention the obvious texture banding problems.