You definitely should keep the new lens.
Mine 24-105mm failed last year. It came with the 5DmarkI and was working fine for nearly 5 years till it went down due to a broken flexible circuit board. I sent it in for service what cost me 150 € to fix it.
In parallel I bought a new 24-105mm and sold the old repaired one for 75% of the market price on ebay.
So my confidence in that lens is not that big, as I didn' used it under harsh conditions for appr. 20.000 shots. Also the zoom ring runs too smoothly after all the years, causing the lens to zoom "automatically" when shooting in a downward direction.
If your lens is in a good shape and it's an "L", you usually should get a decent price for it on ebay, no matter how old the lens is.