In addition to sorting, Eudora has other simple features that make a changeover no-rush questionable.
The main one for me is filters, which are quite strong, not so much the reg-ex and stuff like that, but the ability to make them quickly on-the-fly with distinctions for input mode and manual mode (output I don't care much about). And to review them and find them and modify them. I use a dual-sorting method, the new mail usually goes to special interim spots, and when sorted for permanence the mail goes to another spot. I likely will not go to another program on Windows unless it equals Eudora's filters in strength and imports them accurately, unless Eudora is hurt too much by a new OS.
Eudora is also pretty nice on searching. And it is very stable, both in running as a program and in keeping the mail. (Housekeeping is minimal, you try to keep mailboxes under about 30K email and you keep the memory boxes like In and Out and Trash smallish and compacted occasionally, which is simple.)
Security features are quite good, as is the way it handles things like attachments and embedded pictures. And composing HTML mail is very nice, I often use it as my work editor for other projects, since I also know I can find stuff quickly searching.
The account architecture is excellent for my use, although that varies from person to person and is approached differently in various clients.
The TBird attempts did not cut it so far. I have on occasion also looked at The Bat! and maybe Becky and some others. Good programs all, yet each one has its pecularities and drawbacks, maybe a proprietary format or program glitches or weak HTML composing or viewing. Or mediocre filters. Or this .. or that.
Now granted the original Eudora is not going anywhere, it is finito, and those of us who use it would not mind seeing a new strong alternative (sort of like Linkman to Powermarks) but we know there are a lot of pretenders and a lot of good programs that give you trade-offs. So we wait.
Oh, the support forums are still very good. Qualcomm was never very involved in them anyway, and they took the program to stability and then left. (I don't blame them, the pseudo-free Outlook competition was one element. They are a biggish company with a lot of ups and downs and the revenue model and stream was so-so.) Personally, while the critics love to harangue at Eudora (e.g. you can't set up windows hither and thither with a super multi-tasking look, although the program multi-tasks fine in its limited way) I think still today, a year or two after the stopping of development, it is in many ways the standard for email clients.