I mowed for nine seasons with a 36" w/b, hydro. Each year I had 38-40 properties, at least six over 1.5 acres. I mowed one for a few years that was 3 acres.
Obviously, we have different ideas about "forever." I used my machine hard, it was well paid for, and it made lots of money for me.
Last May, I bought a 48" ZTR. I was able to add only a couple of more properties to my list. From an economics viewpoint, no way can I justify spending all that money for a ZTR. The added productivity is not enough to get the monetary return. I use the ZTR on about half of my properties - the w/b is needed on the others because of terrain, gates, etc.
Remember, the mowing is only part of the time spent on a property. No matter if you have a 36", a 48" or 60" mower on the trailer, you still have to do the trimming and cleanup afterward.
With only five properties, you have a long way to go before exhausting the capacity of your mower.