If Richard Shelby never switched parties

[The Mikado]

He'd retire in 2004, IMO, just because he'd be way out of line with the rest of the caucus by then and Breaux's quest to cash in rather than sit as part of a party that is way out of line with you would definitely appeal to him by then.

He'd probably be hanging out with Breaux as a superlobbyist for the rest of the 2000s.

[The Mikado]

He'd retire in 1998.

It makes no sense for Shelby not to switch, so if for seem reason he doesn't it's probably because he's just going to retire instead of face a potentially challenging reelection.

Republicans won statewide in Alabama after 1998. I see no reason why he would have retired then. He would have scraped out a 1-2 victory in 2004 before retiring or losing by 8 in 2010.

Statewide elections =/= U.S. Senate elections, which are predictably more nationalized and higher-profile.  There's no way Shelby wins in 2004, and he'd face a steep uphill battle in 1998.  

2004 was a great year for the GOP in the Deep South - they picked up 5 Senate seats.  Shelby would be no more lucky.

5 OPEN Senate seats. Those seats wouldn't have gone R in 2004 if Graham, Miller, Hollings and Breaux had stayed (Edwards is a different story). Shelby the Democrat likely would have survived.

No, based on the sample Shelby the Democrat would have probably retired.  Retirement isn't an entirely exogenous phenomenon, Southern Democrats retired en masse in 2004 because the Deep South was lurching very quickly to the GOP during that time.

Yes, this. Shelby wouldn't have run again in 2004. This would be in the aftermath of him probably voting for the 2001 and 2003 Bush tax cuts and for the Invasion of Iraq, and probably for Clinton's Impeachment in 1999, so he would probably be feeling increasingly awkward in the Democratic caucus, anyway, and like I said earlier, he'd see people like Breaux leaving in 2004 or Hollings leaving in 2002 and know that it's time to get out.