You have asked your question in a way that makes it extremely hard to reply to, for (at least in my case) I may not be the best qualified to indicate when my work has improved or decreased in quality. Tenure decisions are usually made on the basis of past accomplishments and future expectations in terms of the quality of one's teaching, scholarship, and service to one's college or university (and, in some cases, contributions to the wider community). In other words, if the quality of my work (and the work of all those tenured) has not increased, then (in my view) the decision to grant me tenure was a mistake. So, I certainly hope that in the case of tenured colleagues (as well as myself) there has been an increase in quality, but I want to pause to suggest that for some (and certainly not all or many) professors it is (in principle) possible that their best work was done earlier in their career, prior to tenure. At least by reputation, mathematicians seem to do their most brilliant work early on, and it may be the case that for some philosophers, their best work might be in their twenties or early thirties. For some of us, it may be that our doctoral dissertations could stand out as a high point insofar as our dissertations are probably more meticulously reviewed than any of our subsequent work. Sure, peer reviewed journals have very high, demanding standards and, in order to be published, our work is subject to vigorous challenges (plus one has to endure exacting, sometimes punishing reviews), but to get published and survive peer reviews is not always as spectacularly thorough as having success at not only writing a dissertation (through seemingly endless revisions under the guidance of three, sometimes maximally critical Ph.D.s) and then you have to defend it in public in front of your committee, a committee that is eager to locate any weaknesses in your arguments or the structure of your project. Still, putting that *possibility* aside, I have met very few (if any) professors of philosophy whose work decreased in quality after being tenured. (Joke: though IF my quality of work has decreased since tenure, I might be quite unreliable in making that judgment).
Important note: I should add that if one does find cases in which there is evidence of some decrease in the quality of work of a professor after she or he is tenured, it is another matter to determine whether that decrease was due to the person being tenured. There might be abundant, other causes, e.g. a harmful accident in the course of rescuing an administrator who appeared to be drowning that caused the professor severe disabilities in her future work, the tenured professor being assaulted by students who were upset that the professor did not give them higher grades or, after tenure, the professor is forced by her soul-destroying administration to serve on endless, pointless committees in which the tenured professor is so crushed and plagued that her teaching, scholarship, and service become a living nightmare (through absolutely no fault of her own) from which there appears to be no escape. And even when there is a case of when being tenured actually causes a professor to no longer function competently, we need to be careful about determining in what ways being tenured caused the decrease in quality of work (for example, imagine a professor is put through such stress during the tenure process that, when she is told she has received tenure, she has a heart attack).