How long do you think it will be before that happens with IBM PC's from the 80's(5150/5160/5170 etc)?
Well, there are several important differences between what I've been collecting and the IBM PC.
First, the IBM PC was designed to be affordable by ordinary people and was manufactured in huge quantities.
Second, when people "decomission" their old PC, they might put it in the basement or the attic or give it to a friend or relative. Or maybe they replace their 5150 with a 5160 and move over the peripherals, leaving a partial system behind.
So the huge production quantity and the fact that those PCs remained around even when not in daily use, makes for a different story in terms of availability for collectors. Plus, we still see warehouse hordes like the Computer Reset thing in Texas currently.
Contrast that to things like SGI workstations. This kind of equipment was made in significantly smaller quantities than anything in the IBM PC or compatible lineup. Even though SGI was quite successful, the absolute quantity of equipment is significantly smaller than anything in the PC/microcomputer space.
These were expensive things and not many people bought them for their personal use when they were current product. They were purchased by companies. When they were no longer useful, those companies typically liquidated them as used equipment or outright scrapped them. Still, they were liked by lots of people, so when companies offered up old equipment, employees or others were happy to take ownership. Once Hollywood got into digital effects, SGI equipment was one of the favored brands, so decomissioned equipment from primary uses was likely to find aftermarket value in smaller companies: contractors, developers, etc. This is still the case with something like an SGI Tezro or Fuel workstation. Even though these products are quite old compared to modern GPU/PC setups, they still fetch a high price on ebay, which tells me that someone is still using them for production work. Most likely this is due to specific software that requires a MIPS processor in order to run properly as the Tezro/Fuel were the last workstations made by SGI that used the MIPS processor. So if you have "tried and true" MIPS SGI software that you are using for production work, the Fuel or Tezro is the most recent hardware you can purchase and still run your software natively.
There's also the size/weight factor. I used to see SGI "deskside" and "fridge" sized computers listed on ebay when I first started collecting. I haven't seen one for a long, long time. The freight shipping alone for a deskside system is probably like $500 at this point. Probably $1,000+ for the fridge sized models. This means that it's unlikely a collector is going to purchase a whole system over something like ebay and since production companies aren't using that equipment anymore either, there's not much incentive for a full-time ebay seller to list such an item. Instead, they list the boards and PSU parts and scrap the rest. This contributes to scarcity.
Get into even lesser known brands like "Sanders Associates", "Evans & Sutherland", "Ramtek", etc., and the things are just downright rare if non-existent.