So a single attack for the offhand?
Well, if you're making a normal melee attack with the pact weapon I'd say yes, because RAW thirsting blade doesn't give you the Extra Attack feature (don't know if this is RAI or not), which is required for making a second off-weapon attack.
However, if you're using Eldritch Scythe, at level 5 you get extra blast, which allows you to make 2 Eldritch Scythe attacks, and it specifically says that this counts as the Extra Attack feature. Since when you use Eldritch Scythe you're still using the Attack action, if you're using a non heavy one handed melee weapon you can make off-hand attacks. Since you qualify as having Extra attack, you can make 2 off hand attacks this way (no Scythe with the off hand, though).
Given this, I'd say that Thirsting Blade is more useful for non-melee characters, which wouldn't take Eldritch Scythe, because ES would scale better and allow you to make more off-hand attacks
Edit: On second thought, you could use your NON pact weapon for Eldritch Scythe with the main hand, and then the pact weapon on the off-hand. This would allow you to use the pact weapon twice in the off hand as a normal off hand attack (since ES counts as Extra Attack from level 5), but this may be beneficial depending on the properties of the pact weapon. I still think that for a melee warlock with Path of the Blade, Thirsting blade is an inferior choice to Eldritch Scythe