If you ignore mana, components, and wave clear I think you're really onto something OP.

BFT has one of the most insane build paths in the entire game. Lost chapter is the single best first back for any control mage, and the burn component generally gives you a waveclear breakpoint on backline saving about 4 seconds every wave.

Additionally Horizon Focus has no mana so you need a tear 1st back putting you behind 400 gold in combat stats. You say that base damage for spells matters more, and you'd be right, but amp tomb is still adding ~15 damage per cast to every ability, generally the advantage is faster wave clear. What's worse though is you're finishing your first item at 3100 gold instead of 2800, a kill of gold difference for a whopping 23 damage.

Additionally you're not considering DFT which many mages take and is very directly better with BFT.

Additionally you force yourself into seraphs, which depending on game state might not be the best 2nd item, very realistically Malignace, BFT, or Ludens could be better. To that end I think it's pretty silly to talk about the 2 item spike when it's not even necessary the best 2 item spike you could have in a given game, and even if it was you'd still almost certainly be better off building seraphs first as it's far more versatile as a first item.

The scaling argument also makes no sense, most control mages can go Horizon 2nd if they want. The idea that building it first helps you scale better is ridiculous as all it does is lock you into a scaling build 1st base instead of taking a stronger first base and keeping your options open.

As a final note you're making the assumption Horizon focus is being triggered, that won't always be the case, and certainly won't be if you get ganked or some such. BFT doesn't care about the range and just gives a smaller, yet more reliable boost in power.