The beauty about this is it was made in 1989 and it still works. Planned obsolescence was not discovered then.
~10y after release, such old computers had use-value. Broken? Repair or use for spare parts.
As they get older, owners understand they have collector's value. And they do - see eg. eBay prices. Even a lowly ZX Spectrum easily fetches >$50 these days.
On top of that: the machine discussed never was a cheap one. Not when new, not now. Not that many MSX2+'s were built.
Never mind that most old home computers are Robust. It's usually mechanics like connectors (easy to fix), keyboards (not so easy) or power supplies (many options there).
It's a guess as good as anyone's, but I'd say the 99% is pretty close to the truth - either in landfill or forgotten in the attic, unused for many years/decades. Only a small number of enthusiasts would continue using them.
Because there are fewer and fewer of them left. Look at something like Pokemon Yellow GBC carts - huge collector item, yet still selling for very cheap on ebay. Why? Because Nintendo made 500 billion of these things and they generally don't break down. Old computers however do, and while it's usually something "simple"(if you know how to use a soldering iron) like a burst capacitor, most people wouldn't even bother.