I know that every year there are still at least a couple of new games for old systems like VIC-20, C64, Amiga, different ZX Spectrum and Amstrad variants, DOS, different games consoles, and so on, some of them even commercial. Is there any data about the market share of modern games for retro machines? Said with other words, for which vintage systems a new game will have the biggest audience?
This might be related to the total amount of units of each system sold, but I think that it is not mandatory to be in a such way. It is possible some systems produced in greater numbers to be not as suitable for games like other systems produced in lower numbers. The existence of emulators also may change the things, by making the numbers of the real system not so important as its actual capabilities. Another factor which I think that have to be considered is the eventual legal issues of distributing a game for an old system if the company which owns it still exists.