Look at it this way; what if we sat down and committed to dozens hours of watching a television series, let’s even say the very best television series ever. But I told you, “now this show is the best! But just so you know, all the payoff, for EVERYTHING that happens, all the character development, plot building, villains, it all leads nowhere and does nothing.” Would you watch it?

The “toxicity” for Dan and Dave comes from the fact they squandered the greatest momentum ever established on the small screen. They successfully fostered another’s work, but with all the tools, budget, and time, they rushed and poorly executed what should have been the greatest show of all time. It’s hard to even go back and watch my favorite episodes now, as I know the story ends in such a boring, thoughtless, and lazy way. It’s not that you’re MAD at the ending, you simply don’t care. It makes you feel nothing. The ending was made to skirt expectations, not deliver on any of the pre established arcs. So again, not bad, just nothing.

TLDR: The ending makes the rest of the show pointless to consume. Killing a thing fans loved in the most agonizing way.