First of all, mana usage has nothing to do with squishiness. Second, go look at Yasuo's stats. Go do it and then compare them to those mages you just mentioned. You'll find that their defenses are very comparable. Yes, Yasuo has a refreshing shield, mobility, and the wind wall. All of that is true. But you know what Yasuo also has that those mages don't? Risk inherent to his playstyle.
They can an all attack from far beyond melee range. They can siege towers and provide zone control. Yasuo has mobility, damage, and the need to get really up close and personal to do most of that damage. Range is tremendously powerful, perhaps more so than any other mechanic in League of Legends. That is why melee champions do more damage, have shields, sustain, and so on. Because being melee puts them at an inherent disadvantage.
And as for the life steal? Please. Warlord's procs once every two seconds, which is every 3-4 AAs if he's not hitting and running. The same applies to marksmen, which as I said, have the safety provided by attack range, as well as their front line to protect them in a fight. They're also squishy as hell and build sustain to mitgate their low defensive stats.
That's the thing, man. Yasuo has ADC stats to do a melee job, so you better believe he'll have a strong kit to get it done.