you can make a 35” conga line that your enemies can’t walk within 1” of

Exactly. Expanding off of this, you could say the purpose of Blightlords isn't to deal damage, but to reshape the board state from a game strategy perspective

You can literally build a wall (and make the corpse worshippers pay for it!) anywhere you want. Blightlord Terminators require an almost absurd amount of focus fire to start clearing them out, so you present a challenge to your opponent that says "Hey, you're either not crossing this side of the board or you're going to waste all of your shooting for two turns trying to clear these guys out"

They also excel at clearing chaff of objectives and then taking the objective themselves and requiring your opponent to concede an objective or ignore the rest of your army trying to take it back

In a straight up kill match, Blightlord Terminators would be bad. But they're an excellent tool for someone thinking tactically to have available for objective games