The elderly man isn't going to be "thrown into the streets" because he already has a home. Again, you have no basis to assume that this is his only source of income or that he doesn't have sufficient savings. He at the very least has family, like the person who made this post.

Property taxes, medical expenses, etc etc. Being retired and owning a home doesn't mean you suddenly stop having to pay for things. They presumably live in shithole america where if you don't have the money to retire when you get older you just die in the streets. It's very reasonable to assume that if a significant source of your income disappeared overnight you wouldn't be in a great place.

And what about the tenant? For all we know they could be elderly and disabled too, only they weren't rich enough to "plan for retirement" by setting up a situation where they can steal rent from someone else.

Even in this situation, it would just be someone poorer fucking over someone else for personal gain. Stocks are unethical too, but if your retirement account was stolen by someone to pay for their medical expenses you'd still feel it unjustified. Landlords as a class should be eliminated, but that doesn't mean literally senile landlords should be left with no safety net.

It's not the tenant's responsibility.

It's an unequal exchange that could end up in the elderly man losing his home due to loss of income. If the tenant had paid the cost of the house in rent, you'd be more justified in thinking this. If the tenant had only been living there for a few years, then it's a different story. You don't get to play "not my responsibility" when one party is directly responsible for the state of the other.