Gilles and Monica already argued along that lines, but here are some more arguments to keep obsolete a separate reason:
The distinction between obsolete and too chatty (or whatever it shall be called in the future) is quite helpful for me as a moderator handling this flag:
If a comment was flagged as obsolete, I first consider:
Did an edit address this comment?
Did another comment by the flagged comment’s author acknowledge that it’s obsolete?
Does the comment reply to another comment that has been deleted?
If a comment was flagged as too chatty or not constructive I first consider:
Is this something that is not a valid comment on any post?
Does this look like criticism? If yes, is it really about the post in question?
Does this look like the author provide some information to be added or considered? If yes, is this actually relevant to the post?
This is already pretty different. So for fast comment handling, it is helpful to know whether the flagger thought the comment to be obsolete or too chatty.
Now, if the case is not as clear as above, I have to take a closer look at the post and all the existing and deleted comments. In that case, it is again helping what I am looking for from the beginning. With an obsolete flag, my focus is on the completeness of the post. With a too chatty flag, my focus is on the scope of the post (and possibly the question).
The worst case if the flag is invalid. With a separate obsolete and too chatty flags, I only have to convince myself that one of these cases doesn’t apply – with the proposed merged flag, I have to convince myself that neither applies.
Now, one might ask why I do not argue against combining flags in general with the above arguments, or even for introducing more flags:
Using a type of flag for communication only works if everybody agrees what those flags are supposed to mean and there is no big grey zone between them. Otherwise choosing the right flag is too difficult or people get fed up with distinguishing and make the “wrong” choice. That’s why it’s good that the not constructive flag gets abolished (and that’s why there is so much quarrel about the NAA and VLQ flag).
But obsolete is different. I have never met anybody who did not understand what it’s good for. The vast majority of obsolete flags on my site are valid. And even on Stack Overflow the accuracy of obsolete flags is very high ([1], [2]). I have also failed to find Meta posts and similar by people who are confused by this flag’s meaning, while I found dozens by an analogous search for not constructive and too chatty. So, this flag is well understood and it works by communicating why a comment should be deleted. And all of this clarity is achieved by a single word – in the proposed interface, you can even spell it out.
I totally concur that removing not constructive solves a problem; I see that renaming too chatty may avoid some confusion; but I fail to see any problem that is solved by removing obsolete.