Blades would want him dead for his past crimes. They don't believe that Paarthurnax has changed, and they would throw it all to waste and rebel against their leader's will, who happens to be the Dovahkiin. You don't actually need a mod to spare or save him, but that requires killing Blades instead when they ask you to choose.
Same thing with Dark Brotherhood. When Astrid asks you to kill someone, kill Astrid instead and your choice opens up another route which allows you to kill the entire Brotherhood, for 3000 gold bounty.
Skyrim is full of alternate (hidden) endings. You don't always have to accept to do you don't want to do. It's not "from point to point" game.