Does Japan have any reason to ever repay its debt?
I would distinguish between ever repaying any particular debt, which it does on a routine basis, and ever becoming completely government debt free, which it has no particularly good reason to do so long as it can service its overall debt load.
At a mechanical level, if you create money to pay debt, you also create inflation and inflation drives up the interest rates on the debt, which makes that debt more challenging to service, until current government revenues can pay debt service and also provide basic governmental services, even with higher taxes.
Aside from the gimmick of creating money to pay outstanding debt, more fundamentally, government debt is about someone voluntarily foregoing current consumption in order to allow the government to spend more than its current tax revenues and other revenues permit, in exchange for greater capacity to consume due to interest on the debt.
What is sustainable in the long run requires projections of (1) future needs for government spending (which is decreased by declining population but increased by a needier aging population), (2) future GDP (which is increased by growth in per worker productivity and decreased by declining working population), (3) the future tax burden (which is a policy choice), and (4) future interest rates (which depend upon creditworthiness and the rates offered by others of comparable creditworthiness in the future).
For example, if you increase tax burden (which decreases the ability of the people taxed to consume) you can afford to borrow more.
Another important factor is the stability of government revenues and expenses. The less predictable these are, the greater a risk there is to maintaining a large debt service load that could pose a disproportionate strain if revenues fall dramatically while expenses rise (as is often the case in a serious recession economy). If recessions are likely to be infrequently and mild, maintaining a fairly high debt load is workable. If recessions are likely to be frequent and severe, carrying a high government debt load is reckless and likely to become a problem.
In this regard, one of the things that Japan's government has going for it is that a lot of the societal needs provided by government in some countries are provided by businesses to a greater extent in Japan. Quite a bit of Japan's social safety net is in the private sector and that makes government spending less cyclic than many countries, which makes carrying a higher debt service load relative to government revenues more viable.
Taking out the right amount of debt also requires government to assess how important it is to make an investment now so as to receive the real world economic benefit of that investment relative to how important it is for taxpayers to have the ability to spend that they would have if the government didn't take on that debt service. Mostly, this varies based upon the benefit that is available from government capital investment at a given them. At some point, for example, the marginal benefit of new investments in transportation infrastructure and educational facilities may decline, but the marginal benefits of new investments in hospitals may rise, as the transportation network is built out and there are fewer younger people and more older people.
In practice, predicting these various future scenarios requires considerably talent and has big impacts, which is why we pay economists and other finance professionals good salaries to help us determine these things.