Do you own gear from defunct brands?

Yes, Amber, DCM, Dynaco, EPI, Great American Sound, Infinity, NY Audio Labs, Phase Linear,  Rectilinear, Snell ... more!

Yes, a Classé preamp (the status of Classé is unclear) and an Auralic Aries G1 streaming bridge.

That is an interesting question.

I can’t think of any. I have a Marantz integrated amp from 1972. While I have had a few components that were from companies that went out of business I had traded them in long before for a better sounding solution... example, Auralic. 

I think I have a tendency to subconsciously consider the company when I purchased. For much of my adult life I have been in charge of selecting software (as a minimum $100’sK usually millions). Performance and the continued existence are critical... typically there will be a dozen companies producing a particular kind of software and only two will survive. Choosing the wrong one would be a complete failure on my part. So, maybe that crept into my decision making. Also, in this field, performance is critical for long term survival of the company. 

Carver tuner, Boston Acoustics speakers, 

All the rest including Dual still alive and kicking in one form or another as far as I know. 
 

Is Goldring still around?  That cart still on Dual I bought in 1981.