There was no way that I could kill Paarthurnax. After everything he did for me and how little the Blades did for me, they wanted to tell me that he deserved to die? The way they treat dragons is no different than the way the Nords treat all other races, which is also no different than the way the Thalmor treat all other races. So in essence, the Blades, like the Nords, are hypocrites. Paarthurnax is an awesome character, he's helpful, and he did more in one quest than the Blade's do for you in every quest involving them!
The Blades questline isn't that great anyway. They give me help to kill dragons. Riiiiiiiiiiiight. SoI have massive amounts of health and Magicka, an invincible Argonian follower, a powerful (and hot) vampire follower, the ability to call a dragon to aid me in battle or summon one from the Soul Cairn, a crapload of shouts and powerful weapons, and spells that allow me to shoot lightning or summon Daedra, yet I need some dragon-hating crusaders to help me out that end up dying or running away (at least, for me they did)? Uh, yeah, thanks but no thanks, I think I'm good. Then there's Esbern's potion, which is glitched anyway. And finally, why do I need to recruit followers into the Blades anyway? The only followers I really use are Derkeethus and Serana. The former stays at Darkwater Crossing whether you recruit him or not, and latter can't be recruited (smart girl). Other than that, they can't really help me out.
And let's see what the Blades have done for me compared to the Greybeards.
Blades:
Stole the Horn of Jorgen Windcaller just to meet me.
Had me kill a dragon I probably would've found and killed anyway.
Had me infiltrate the Thalmor Embassy only to find out the Thalmor know nothing about the dragons and are looking for Esbern.
Told me there was a shout that can defeat Alduin.
Greybeards (including Paarthurnax):
Tought me how to use my Thu'um.
Granted me the full Unrelenting Force shout and the first Whirlwind Sprint shout.
Helps me find other words of power.
Told me how to find the Elder Scroll.
Instructed me to try and capture one of Alduin's allies.
Allows me to meditate on one of three words of power at a time.
Actually helped me instead of trying to help themselves as the Blades did.
Overall, the Greybeards actually helped me in the main quest and told me what I needed to know, whereas the Blades only told me and had me do what benefitted them and what they thought was best, not what was actually necessary. The Greybeards are the better faction, Paarthurnax deserves to live, and Delphine and Esbern deserve to die, so I made them unessential, turned them hostile, soul trapped them, and enchanted my Daedric Swords using them. That actually made them more useful than when they were alive!