Recently I threw away an old (and only) TV-set and an old PC's system unit. In the past I did the same with an old music center, big CD's collection and other equipment. It also makes sense to get rid of some furniture, but it's a lot of work.
But as for keyboards, my PSR-3000 was in a case for 5 years with a broken contact strip. When I had the same problem with S950 I had them both repaired, then PSR-3000 was back in a case. Now my S950 is in a service again (it takes more than a month), so I play PSR-3000. Looking through ads I see that prices for used PSR-3000 are rather high, so it would not be that simple to get a keyboard like this just to use it as a backup. So right now I'm glad that I kept it. Also, when it comes to old TV-sets, printers, computers, it's obvious that they can't be used nowadays, but a lot of keyboards are still useful.
It's indeed inconvenient in terms of space to use more than one keyboard, but in my case that would involve paing extra $2400 to have all what need in a single unit instead of having several instruments. Again, having a backup can be crucial.
Anyway, keyboards are not pianos. Several of them can be stored in a wardrobe with clothes and it won't make much difference.
I think, it's possible to go too far in any direction.