Regarding issue number one. There is a significant level of nostalgia involved with my desire to add a console to the collection. That nostalgia can be either due to my previously owning that console, or my desires to own it as a child. If that nostalgia isn't there, I don't seek out that system.
Regarding issue number two. There is the issue of a consoles library of games one has to consider. If a library isn't diverse enough with its own exclusives that stand out enough, I don't see a need to add to it. Especially if there are cross platform titles that are superior on the consoles I already collect for.
As an example, DOOM. I have DOOM on my Jaguar, PC, and Xbox 360. I intend to get it for my Playstation down the road, as well. I would also like the N64 version due to it being so different in comparison to all other iterations up to that period. However, I won't use DOOM as a reason to get a system I don't have an interest in. If I already collect for a particular system, then it makes sense. Otherwise, why would I bother? The original DOOM on the SNES, 32x, Jaguar, Saturn, 3DO, and Playstation are ALL identical, minus a few differences due to limitations of the system it is released on. However, they all play exactly the same. Here is a video showing you the differences between them, along with the original DOS and Game Boy Advance versions.
An example of a console I do not have a desire to add is the 3DO. I remember when the system was first released. It was a whopping $699.99 for a bit there. And people were flipping out over the PS3 price tag a decade ago!
The 3DO just does NOT have a diverse enough library to garner a reason to add to my collection. The US/NA release library is only at about 162 total titles. A LOT of those titles ended up on the Saturn or the Playstation, if not both. I own those two systems. There was also a large amount of FMV (Full Motion Video) titles. If anyone remembers those...you know why I don't care to collect them. Sure, the system is intriguing, but it just isn't intriguing enough to garner the purchase.
Now, this is the third dilemma comes in. I want to collect consoles for the idea of "preservation". These consoles are literally dying. I am not saying that Atari 2600 you have in your closet is a rare gem. What I am saying is that as time passes, the systems inevitably become more and more prone to failure. The internals are generally easy to fix on the older consoles, but newer systems tend to have a much more intricate design. We also see so many videos on social media where people are outright destroying these machines, whether functional or not. By destroy, I literally mean destroy. There is also the issue where some are gutting the consoles, and repurposing the casing. Unfortunately, there has been times when the internals of the system were still functional. Another issue regarding "repurposing" is when a perfectly good case to a retro console would be more useful to house internals of a working console that has a broken case. I appreciate the idea of repurposing/recycling old stuff made new, but there is an aftermarket for broken systems and cases.
Granted, one could look at the overall "issue" I have presented as a non-issue with the popularity of flashback style consoles, and emulation. I collect those flashback style consoles to help fill in voids in my collection. I also utilize emulation to enjoy the long lost games of yesteryear that are considered rare, or too expensive to purchase on my meager wages. If I come across these various systems...whether mentioned or not...at a reasonable price, I will snatch them up. This is only when a deal is present, though.
In the end, we all collect these classic gaming machines for a variety of reasons. Ranging from nostalgia, preservation, or just the joy of collecting these oddities of entertainment. Why do you collect, what systems do you not collect, and why?