Upgrading doesn't necessarily mean lighter for the sake of lighter parts.

Upgrading can mean more reliable, more efficient, better fitting, etc., etc.

For example, at 235 pounds, I need a better headset than the one that comes on most factory-assembled street fixies.
Not a lighter headset, but a heavier-duty headset.

Further, I ride clipless pedals, but, because I commute in all kinds of weather and all kinds of traffic, I want them to work elegantly in all situations, both clipping in and clipping out.
That means spending a little more for doube-sided Shimano SPD MTB pedals than I would for a bargain one-sided set of pedals.

I don't care much about top-end speed, but I do value fast-accelerating wheels that I don't have to baby.
Cane Creek Track wheels have astoundingly low rotational inertia and they take a beating with a heavy rider and mainain their trueness.

For more than an hour in the saddle, a Brooks saddle does the trick for me.

I don't like fixing flats, so I ride the most flat-proof tire I can find, in back, an Armadillo.

I don't want my front tire going out from under me in lousy spring and fall weather, so I ride the best all-condition flat-resistant tire I can find, in front, a Conti 4 Seasons.

I like a more silent, reliable chain, so I ride an 810 instead of a 610.

I think EAI makes a better cog than anyone else, and so I pay a little extra for EAI cogs.

I prefer a single-pivot front brake over a dual-pivot brake because I don't want to go over the handle bars in an emergency stop.

So upgrades can reflect aspects other than weight, and, because I have a better-fitting, better-handling, faster-accelerating, more reliable and more comfortable bike for it, I consider the money well-spent.

That said, Da Chick put together a remarkably good bike in the Bianchi Pista, and it served me well while I learned more about these fixed-gear critters.
If I never up-graded it, it would still serve me well, except for the headset.