I have owned a BX22 with a mowing deck and learned after 15 years that it was just not a good grass-cutting machine. I had to remove the loader and the backhoe to mow, and remove the mowing deck if I wanted to do work with the loader and backhoe. I finally purchased a ZTR for cutting the grass, and when I upgraded from the BX22 to a BX23S, I didn't get a mowing deck even though it is a lot easier today to remove the backhoe and loader than it was with the BX22.
I wouldn't consider selling the backhoe from your present tractor, but I would suggest that you sell the mowing deck, and put that money toward a ZTR for grass cutting. I would keep the present Kubota for digging and loader capabilities that the BX23S is sadly lacking. For me the B was overkill and the BX is just a little short sometimes. If I need a larger machine, I have a friend that will bring his bigger machine over to lend a hand. I see my BX23S with its factory cab more as a snow-moving machine than anything else. It is small enough to get around the property and large enough to handle 99% of the snowstorms that we get. For that 1% of the blizzards that I can't open the road to our home, the neighbor that has a truck-mounted snowplow will make a pass to open the road, and then I can handle the rest.
If you have 30K invested in the B and you believe that it is only worth 10K then keeping it makes even more sense than investing $23,000 in a machine that can't do 75% of what the present machine can do. Spend $3000 on a ZTR, and if you get $1000 for the mowing deck, it will only cost you $2000 to have the best of both worlds.