Maybe so, maybe not. Doof was fairly publicly against Kronos and Judge Doom. If a major corporation we hate falls once and we snap up the pieces, that's just corporate life. It's not like Syndrome was that competent a CEO, or at least that's what the other kings will tell themselves. If it happens twice... Well, we've also had beef with ENCOM and Toffee for a long time. They're gonna start worrying if they're going to be third.On the other hand, we know from the Fall of Kronos we can just snap up the bits that fall off and nobody will mind.
That is to say, I don't think Toffee himself will feel threatened, but he's probably going to see us as a threat to his foothold on earth and respond appropriately, lest it become another entry in our track record.
That being said, I am still absolutely in favour of taking down Judge Doom, I just think it's important that we go into it knowing that there will be consequences, both in how the landscape changes, and in how the other kings will see us. After all, has any king other than Syndrome fallen?