Let's assume your theory is correct - what kind of business situation would we be in if fans are not buying the official gears? Adidas would most likely lower their bid for such crappy rights.
United will make, at most, 15% from every kit sold (I've seen a lot of speculation around whether it's 15% of the total price, 15% of the total profit or 15% of Adidas' cut. I have no real answer as to which it is, though someone else might). Usually, though, those payments won't kick in until a certain amount of kits have been sold. For all we know, that number hasn't been reached and kit sales are, at this point in time, completely irrelevant for United's ability to buy players.
And how are the rights "crappy"? Other clubs have exactly the same problem with knock-off kits, and United is by far the biggest mover of kits out there. 2,75m official United kits were sold in 2016, that amounts to £220m. I don't know how much Adidas makes per kit sold, but assuming £20 (high? low? feck if I know) that's £55m on kits alone. Then there's all the other apparel sold, and the fact that there are millions of United fans out there who will now prefer Adidas over the likes of Nike and Puma. It's safe to say that Adidas rakes in way more from sales of United branded products, and other products bought by United fans, than the £75m they pay us.
In short; Shirt sales will have little to no effect on our spending ability, and Adidas makes truckloads of cash from the deal. There's also 8 years left on the deal, so I'd say it's a little early to start worrying about Adidas deciding we aren't worth it (we are. Very much so).