It IS how it works.
The $7500 credit is all but completely phased out already for Tesla (it's only $1,875 right now). And yet they're STILL outcompeting the other Plug-in electric cars in the US by a large margin (13,150 Model 3s in August versus 2500 Prius Prime, the next most-sold plug-in vehicle). That is my point.
It's an advantage to conventional carmakers who wait to the last minute to invest in electrification, after someone else has done all the hard work, and just integrate some off the shelf electric drivetrain. It puts those who actually invest in a full, cleansheet efficient electric drivetrain domestically in the US (like Tesla and GM) at a big disadvantage versus foreign firms or for multinational corporations like the Volkswagen group who have multiple subsidiaries like Audi, VW, Porsche who each get their own group of credits (even though the Volkswagen Group's MEB platform will be available to all their subsidiaries).
It makes no sense and incentivizes the wrong behavior: advantages foreign firms above domestic. Advantages latecomers over those with the foresight to invest early (Tesla and GM, for example). There either should be one big pot of credits, or the credits should be sunsetted on a global timeline (instead of by number).
And until conventional cars are paying for their full externalities from carbon emissions (and others), they are being implicitly subsidized and an EV credit of some sort (although imperfect) will be needed to level the playing field.