I disagree. The point of a BF Sword is that you sacrifice the interim items in order to get it. If you want early-game AD, get a Brutalizer (which, IMO, is extremely good for its cost). If you want to rush a power item, that's why BF Sword is such an expensive one-shot item.
If BF Sword were built up from lower damage items, it would reduce the timing disadvantage that you're supposed to be putting yourself into for a stronger later-game. It's balanced the way it is because if you commit to going for BF Sword, you'd better be able to farm it up quick, otherwise you're just going to feed.
This is a terribly weak argument. You are basically saying 'It's that way because it should be' and not providing any compelling reasons why.
What does this 'timing disadvantage' accomplish? Nothing. When you go for something like a Brutalizer, you're getting it because of the bonuses or to build it into something. That piddly 25 AD isn't why you're there.
What does making the BF Sword build off of Long Swords accomplish? In my opinion, a lot.
You get a more gradual increase, making the cost a little easier to bear and giving a more immediate return for your money.
It encourages more non-linear building; you get a long sword, then next time you reach the shop, you have more options to devote that long sword to, whereas if you're going for a BF Sword right now, you have one option: Don't spend anything.
I also wouldn't be averse to the pickaxe, blasting wand and crit cloak getting a similar treatment, but their cost isn't nearly as severe so it's not quite as bad.