When I can replace it with a similarly capable vehicle. 6MT, equivalent towing capacity, similar interior capacity, not meaningfully externally larger.

Unfortunately, I don't think that that will ever exist again. Certainly, today's X5 doesn't fit well in my garage. (I can comfortably park 4 cars diagonally and one in the drive aisle if there aren't any that are too large. The E53 is on the outer edge of what can be diagonally parked.)

__________________
2011 M3
2006 Sierra 2500HD 4WD LBZ/Allison
2004 X5 3.0i 6MT
1995 M3 S50B32
1990 325is
1989 M3 S54B32

Hers:
1989 325iX
1996 911 Turbo