Unless you "need the money", I think you should keep the Canon 200-400. I look at capital budgeting for a good chunk of my real life, so for what it's worth, this is my rational.
The cost of keeping the lens is not $12,000. The cost is the opportunity cost of the $12,000 so unless you need the money for something that will earn you money, the cost of the money is just the interest you would earn by putting the money in some liquid investment. At 3%, that's only $360 per year. You will easily spend that much money in rentals per year.
Your residual value will change little to none at this point because lens depreciation is stepped. Meaning, you take a big hit once you open the box and then it levels off.
I have a 200-400 that I use very little. I use it for eagle photography once a year and for my young kids in field sports. The cost and hassle of renting isn't worth it for me.
So...unless you need that $12K to buy the other lens or you need it to fund some other capital item, it makes no financial sense to sell the lens and rent.