But instead of the Ranger stepping up next, it is the Warlock who must fill in the breach. The Ranger can't go toe to toe with things like ogres, etc., because he can't take hits. So even though the Ranger has better melee weapons at his disposal, it is the ill-equipped (in terms of weapons) Warlock who has to interpose himself between friend & foe. And his powers & abilities do not lend themselves to that role. Suddenly, he goes from a ranged combatant to recon with, to a guy swinging a hammer with his MBA...just so the squishy Ranger can do his thing..
Don't you have any Blast attacks?

I don't have any issue with Infernal warlocks being tough; it's thematically appropriate and they tend not to have the greatest AC so it evens out. But I do think that melee Strikers often seem a bit too squishy, and don't get much to balance against the advantages of being a Ranged striker,. Indeed Ranged strikers often have better AC & Ref due to better DEX. We had a melee Ranger in one campaign with CON 10 and AC 13 (Hide) at 1st level, who was constantly getting knocked out.