There are currently 420 tracked items in the rip chip boxes, with an unknown number of titles as well in each of the 3 builder boxes. That's a lot of stuff to unlock, the problem isn't that there's not enough. The problem is it takes way too long to earn enough to buy it all, because the rewards are way too low. Again, I have 123 hours and I still have at least another 10 hours before I'll be able to buy every single thing. That's an insane ask for players and is more time than I've spent on 99% of the games I own, most of which have well over 5x the amount of gameplay content.
I already do plenty of co-op. Getting an extra 20 rip chips per challenge is not going to fix the issue of an over-inflated rewards economy. Getting an extra 45-75 without even having to bother with co-op (and therefore getting 65-95 extra with co-op), would still not be quite enough in my opinion, but it'd be a much better answer than adding tons more stuff to buy.
The core gameplay is fun, when it doesn't break apart and send you flying off your board for no reason. But the grind for rip chips and boxes is one of the most dull and repetitive things I've ever experienced in a game. Like Akelaphobia said, games should not be balanced around the wants of people with hundreds of hours spent playing, when they're the only ones who've run out of things to do.