Depends on what you want to do with it. If you have similar hardware that will do what you want and the price is too high, it may not be worth it.

The failure patterns of old motherboards is something I would like to know more about. In my experience (which is limited) the most common problem is not cap failure but worn-out PCI/memory connectors. Of the BX boards I've had, all eventually had this problem, where you have to repeatedly insert/remove/adjust the module/card to get it detected properly. It's frustrating. Cleaning the contacts with air duster/alcohol/brush helps but only so much. This is why I nowadays prefer to buy server hardware from that period. It's much more robust.

At the moment I don't really need it, but just in case if these are vanished totally in ten years and I still have working ISA sound cards, then I would like to have some of these as spare parts. But don't know if it is really worth having because of that reason.

Best would be if one could buy a new retro computer after ten years with FM sound card and adjustable CPU speed. With FPGA this could be done, but not with my knowledge.

There are still used boards that can be had at the $20 to $40 shipped range, just got to know where to look and take the time to search through but given that you live in Finland it won't be as easy. It is a lot worse when one is trying to buy a slot 2 xeon board.

Like with anything old... you can get lucky and get something for peanuts or if you really want something, put down hard cash. You can wait till you get good deal or BIN. It's just a matter of what and when you want it.