Should I wax or polyurethane an old pine floor?
You already answered why; the poly that was put down is peeling off! My parents tried to use poly on a new pine floor when I was a child. It peeled and chipped and wore off in areas after about 2 years. I suspect (now) that the difference in rigidity was the primary reason for the separation of varnish (hard) and pine (soft). Surface finishes like varnish dry on top of the wood. I'm afraid I don't know why varnish would be worse on an old pine floor. Actually I would be inclined to believe the opposite because old pine should be harder; but it's really just a question of which is worse new or old pine?

I believe that Rubio Monocoat is probably derived from tung or linseed oil; it contains a drying accelerator (aka siccative); perhaps they use a manganese salt like manganese acetate. I'm not familiar with Rubio Monocoat so I can't say anything about their quality (sounds interesting... might work). There are many possibilities for what this material could be, but it's not wax.

Modern floor waxes are actually not true wax either; they are (typically) acrylic polymers. But that doesn't really matter, the point is that floor wax does work on pine, or at least it won't peel off.