Could you imagine back in 1999 if Britney Spears knew that BOMT would immediately change the world of pop music forever?
Do you think that an artist knows when their work will later become iconic, or is that later decided once the general public dictates that?

No I don't think they would know. No one can look into the future.

Could you explain a bit further how BOMT changed the world of pop music forever?

Do you mean by introducing Britney Spears to the public?

It must be that because aside from it being iconic, it didn't really affect the pop landscape as bubblegum pop had already been around through the Spice Girls and R&B infused Pop was also a thing (as evidenced at this originally being made for TLC). It encouraged more Lotita-style popstars but that's about it.