That's true, and hard to argue against. I guess you could take the human feat to get +1 con and con save proficiency, but that hardly seems amazing. Better would be to take War Caster as your level 1 human feat. That actually seems seriously good the more I look at it. You can also dual wield and still cast spells so long as they don't have a material component.
I am struck by how much you don't need armour though, if you're going dex based. My plan would be to just stay out of melee until I actually got given my pact blade at level 3. Then learn to cast mage armour at will and have AC 16, scaling up with dex to 18 at level 8. Maybe eventually you end up finding a nice mithril chain shirt with an enhancement bonus on it and wear that. If you're going to have dex 20 anyway, why spend a feat learning to wear armour?
There are arcane focuses, of course. In 4th edition you got magic ones of these that helped you hit with spells, giving pluses to hit and damage. I expect we'll see these in the dmg and very possibly some kind of warlock blade-focus thing, as there was in 4th. So then you'd have one hand that held your weapon, somatic and material components all at once and you could do what you liked with the other hand. Warlocks might well end up with the ability to make weapons too.
I expect to see a blade warlock going around with a magic spell focus in one hand and a magic shortsword or rapier in the other, made into their pact blade. I worry that anyone making solid plans for their off hand of any kind will find themselves wishing they could still get pluses to hit with spells at higher levels.
If there aren't magical focuses for arcane casters, but there are magic weapons, then right there you have the point of blade warlocks. You get to chop at people with a +2 flaming shortsword while the other guy's eldritch blast has no enhancement on it. Even then you're struggling with having to get two stats up, which is about equivalent to a +1 weapon in effect. It gets weird at the point where you can fire 3 entropic blasts but you add charisma to blade damage, especially since you can get weirdness like haste to think about. It's all very hard to figure out until we actually see some dungeons.
So my plan for a blade warlock would be something like: S8, D15+1, C14, I10, W8, Cha15+1. War caster from level 1 so I get to keep my hex running, hopefully. I don't think I'd dedicate fully to either chopping things or blasting them but aim to eventually have both dex and cha up to 20, perhaps alternating which one I raised. I'd probably try to pick up some nice dex and cha-based skills, maybe starting out as an urchin and taking beguiling influence at some point, or entertainer. That would give a load of utility in combat and out of it.
