Just wanted to address this. As long as analog hardware is considered collectable, and at this point I see no indication that this won't be true. The law of diminishing returns isn't going to apply.
Anecdotally I have three hardware synths:
I bought a Memorymoog for $1000 when I was a kid used. It sold originally for around $2300, they regularly go for 6k broken and up to 25K depending on whether they have the Lintronics mod or not etc.
Bought an Oberheim Xpander around 2008 for $2,300. It sells for about $3,800 to 5,400 on eBay now.
Bought a Metasonix Wretch Machine For dirt cheap in a package deal. There's exactly one on eBay right now, for $4,000.
This isn't to brag, but if you're looking at the OB-6 and thinking that a true analog with the amount of integration into a DAW that it has is going to go down in value you're mistaken. I don't personally believe this, but a lot of people think true analog oscillators, filters and distortion will never be exactly emulated in software, and with that in mind, hardware that uses those components is not going to diminish in value the way a DX7 did.
I compare this to my soft synths, and some are just horrible investments, they're fantastic when it comes to sound, but they do not offer much backwards compatibility, (new versions that don't load where the old one did etc.), and there's no guarantee that the company stays around in case OS X or Windows versions break some old code etc. Had I bought into digital hardware synths I would agree, since there's no logical reason a digital synth can't be emulated with 100% accuracy.
