There was a similar conversation in the Thought Erasure thread. There I argued that I didn't think there was much of relationship between the playability of Duress and Mental Misstep. I stand by that, but I think this thread raises some other interesting topics.
I think I tend to take the opposite approach to running cards/building decks and I wonder if there wouldn't be some benefit to trying that here.
Basically, a question is posed in the form "Discard effects are bad. [conjecture as to why]. How can we make them good?" While I think we'll find that discard effects are still pretty reasonable, it's a problematic question to ask, because there might not be an answer. If you had asked "Vizzerdrix is bad because it's strictly worse than many other creatures. How can we make it good?" ... you sort of can't, short of changing the rules of the format so much to be unrecognizable. I think there are a few questions you could be asking instead, which may lead to more actionable answers.
What do I want [discard] to accomplish, and what is the best card or strategy for accomplishing that?
While Duress was classically a storm-deck staple for punching through combo-control decks, if you're specifically trying to beat a slower deck with Pyroblasts and Flusterstorms then Defense Grid just does that better. On the other hand if you're looking for action on turn one, in a deck that primarily cares about the stack, maybe the first Duress does that better than the 6th cantrip ... or maybe it doesn't, but at least you know what to measure now.
In what situations is [discard] already good, and what sorts of things make those situations more likely to happen?
There's a reason you're asking about discard in the first place, right? There must be some game state where you really like it ... a Thoughtseize can punish greedy keeps, or provide information on a critical turn, and it's a cheaper way to help your cards resolve than playing multiple threats ... so making Thoughtseize better could involve playing cards that make those things more common, or more important ... Decks with Wasteland are more likely to be able to capitalize on punishing a one Mox//one Land keep. A control deck with a weak combo matchup may benefit more than usual from the information advantage. Considering the broader metagame, if the decks that discard is good against just aren't being played, it's possible the only thing you can do is wait until they are.
What's the best possible deck I could run, if I accept running/building around [discard] as a handicap?
Recognize that when you see a card you like from a spoiler, or you have a pet card you've been shoving into every deck for years, this is the question you're really asking while deckbuilding. There's nothing wrong with doing this, I've done this many times myself, and sometimes it's the best way to discover something truly broken before anyone else ... but if you don't recognize this is what you're doing, you're sure to get a lot of "why haven't you cut Kess, Dissident Mage?" sort of advice from everyone you talk to your deck about. Many people don't want to believe they're running a particular card just because they enjoy it, but if they don't have a good answer to one of the other two questions, that's what's happening.
So ... why do you want to run more discard in vintage right now? Do you think it fills some useful role in an existing deck? Do you think it's undervalued in a particular matchup? Do you miss playing with discard spells and you want to maximize your chance of winning while getting to cast them?