Death Pact doesn't feel like an ultimate skill for me. I'm not talking about which skill is the most effective one, I'm talking about game design. Literally just eat a creep and gain some of their HP and damage, sounds like a very underwhelming ult. Compared to Burning Army which is flashy and became what people remember when there's Clinkz: "He got Burning Army be careful". IMO Death Pact should be a basic skill, or maybe a Shard/Scepter (I know Death Pact used to be Scepter).
I don't really know much / remember about why they replace Burning Army, but if I can guess probably because it's OP. But this is definitely not the solution. And this is probably just me, but the game design in recent years has becoming more messy, like adding a LOT of questionable and unnecessary stuffs.
NOTE: I'm not an old Dota player, my first game is on 2019. So I don't really know his previous skillsets and patch notes.
Source: https://www.reddit.com/r/DotA2/comments/uda0cp/maybe_unpopular_opinion_they_should_bring_back/