A) There are better preamps. OK, “better” is in the mind of the player. But, the Fender preamp is not a circuit that cats go out of their way to retrofit. If you like it, that’s great.
B) Fender does have some very desireable design aspects, particularly on the 5 string neck. Sadowsky, for example, does not offer 18 mm string spacing at the bridge, and thus, has a pretty wide neck profile to fit the 19 mm spacing.
C) You may think your $2000 Fender is expensive; but, have you priced the “boutique” J Bass builds? They are considerably more expensive.
There are multiple highly rational reasons for modifying a Fender. But, OOC, why do you care what other people do with their basses, if you are set on what you do with yours?