I know Canada doesn't like retirees, in fact the older you are the harder it is to get a visa. The problem is, Taiwan like Canada have universal health care, so if someone gets sick the state helps pay for the bill. The problem is, older people are more likely to be a burden on the society when it comes to medical stuff and that puts countries with a universal health care system off. I just think this may be one reason why Taiwan doesn't offer a retirement visa.
Also, if you notice Taiwan is already full of old people, you know especially when you go to a hospital there's nothing but old folks there (I mean except for the doctors, nurses, etc.), so it's likely if more old folks were to come to Taiwan they would most likely be in or out of the hospital, straining the already strained NHI.
I can understand countries like Malaysia if they are just looking to make money and they have no health care of any kind that the average folk can afford. I suppose one thing Taiwan could do is not allow people on retirement visa get NHI unless they can prove that they are in good health.