Japan's population is shrinking as a whole but still growing in cities (especially Tokyo). You can already get a house in the countryside for free : https://inakanoseikatsu.com/category/不動産物件/無償譲渡（0円）の空き家物件/
First of all, you still have to pay to the real estate agent for completing the required transaction regulation. You also have to pay tax on transaction and yearly.
Some house owner actually pay you on transaction for the hope of freeing from above yearly tax cost. They are so desperate to un-own the asset. It has practically no value yet, the tax is calculated like it had a value once upon a time.
You must also know that in Japan, you can't abandon the real estate. Once you own it, you must own it until you transfer it to the other willing to own it. It's not an asset, it's a debt, burden you for the rest of your life.
If you actually purchase these free house and pay the tax and transaction cost, it's not over. Nobody want to rent it. And you can't practically live it. If it does, it won't be sold in zero JPY.
Water, Power and Gas pipe need replacing as well as floors, walls and ceilings too.
If you repair all of these, it's actually cheaper to scrap and build a new house. It will be better by all means, it meets all the recent regulation of anti-earthquake, anti-typhoon building standard. These regulation exists because of the reasons and you don't want to live in a house that doesn't satisfy these regulation for your life.
The land itself has no value. It's located at very inconvenient places. The best case, you have to drive hours just to go to the local store. In most cases, it's located at small island so you have to use ship instead of car.
I think the real “string” is economic, which is the reason these houses ended up abandoned in the first place. They’re almost entirely in places with fuck all going on.
That is if they actually own their homes. In Japan, large potion of current working generation can't afford to own a land and house. So the real estate is not going to be the largest retirement asset for the most people in Japan.
That and the fact Japanese population is shrinking so there will be less demand for it, I'm wondering the real estate value will continue to raise like it currently is.