Ok, that is their choice of course, but this is not about making money (from the numbers I found they cannot be making much of that) or they would be working on something else?
Demos are an outgrowth of the software-piracy scene. Most developers of non-grey-market software expected to be compensated for their work.
In fact there was one case where Microsoft used open source to destroy an entire software niche on Amiga: they acquihired the developer of Bars & Pipes Professional and got him to open source it, destroying the market for Amiga sequencing software. When you consider that musicians have kept their old Atari STs going until well into the 2000s, that market was bigger than you might have supposed.
