Personally, I don't think Josh Johnson was ever truly healthy last season. When he pitched, he always looked like he was really laboring to me. Not the free-and-easy stuff he had in Miami.
"Life is what happens to you while you're busy making other plans...." John Lennon
But if he accepts the offer they can't justifiably sign another catcher, like say McCann, so they might not be able to afford to trade him.
And meanwhile while they're waiting to see if Salty signs the offer and then taking time to work a deal (which isn't guaranteed to be completed in less than 24 hours with his new price tag) somebody else can move in on McCann.
I'm not even saying you're wrong here that offering him is the right move, btw. I'm just countering your claim that there's no risk. There is always risk.
League Team years Record Wild Card Division Pennants Titles MSL San Diego Padres 2034-2059 2,217-1,995 1 6 3 1 TBL Arizona Diamondbacks 2005-2018 1,216-1,053 1 9 6 3 TSSL San Diego Padres 2015-2021, 2024-2028 1,017-928 0 7 3 2 TSSL Texas Rangers 2029-2033 396-414 0 0 0 0