But for what it's worth, birds are the closest living relatives to dinosaurs and we have no trouble digesting birds for nutrition. In fact, chickens are the closest living relatives to Tyrannosaurus Rex. So we would have no trouble at all roasting a T-Rex and consuming it for nutrition were we able to hunt one.
Size: “thriving” is incredibly vague. Would you say plant life is “thriving” in the environment of the Blue Whale?
Abundant: how abundant? I have seen no estimates for primary production in the Cretaceous, as googling gave me paywalls.
Third point: Even plant species evolve to fit their environment, so for a discussion about plants that exist today (and, worse, which humans selectively bred to be full of calories) it doesn’t matter how much other life there was c. 100 MYA because the plants around now didn’t evolve for what was around 100 MYA.
Fourth point: depending on which era of dinosaurs you’re referring to, plate tectonics radically shifted global geology since then, changing how much coast there was, how much land was desert, where mountains were, and global ocean currents, so even if the plants and animals were the same as now, the total living biomass would be different.
Fifth point: the plants can still “thrive” even if they’re no longer what we consider “nutritious”. They can also still thrive even if something else does. Grass/cattle comes to mind, although the reasons are almost the exact opposite to this scenario.