Whether Dark Heresy is built to use currency is a matter of opinion, one I very much disagree with you on given that its predecessor Warhammer Fantasy Roleplay was designed with hard currency in mind, and functioned fine for over two decades. It currency system was more complicated, even including the taxes for specific cities (which I really miss in settings like DH)
There are /flaws/ with using just hard currency, but I really don't see why using INFL as a way to requisition equipment and than giving characters actual money to use in the adventure is infeasible (which is what you appear to be saying). Its just a matter of taste, arguing that one is completely unworkable is just a farce.
Old thing worked adequately for its time, therefore there is no reason to improve it! This would be like if the field of psychology never moved beyond Freud because it worked well enough at the time and had a decade or two of inertia behind it. As hard as this may be to realize, there have been improvements in the "technology" of role playing games. It's fine to use old technology as a baseline, but its important to recognize when it's time to change things.
So basically, yeah, "old thing worked okay" as an argument that ignores the flaws in the old thing and the reasons for changing it is not a good argument.
Your entire argument revolves around your assumption that the influence system is an improvement, which is not shared by everyone here, so its a really terrible argument because its entirely subjective. Its a change, but whether its an improvement is entirely debatable.
If you've been paying attention I'm actually not advocating a sole 'hard currency' system, I've pointed out the flaws in that for a setting of this nature many, many times. I'm also against a pure 'roll to acquire' system because I think it detracts from the game in terms of immersion. I know many players that enjoy playing out some parts of downtime, which include actually 'contacting' their contacts to get something or talking to the shop keeper to haggle for something. Using solely influence would make all these things redundant.
Admittedly, not everyone does enjoy that, which is why I'm trying to find a balance. But your coming at me as if your opinion on the influence system being an improvement is apriori, which it isn't. Its a matter of taste.