Not disregarding some of the impact Janet Jackson had on Britney's career, but the fact is that 'Toxic' was NEVER written for Janet Jackson.
In that widely circulated quote, Cathy Dennis clearly said about Janet, "I went over there to write with Janet Jackson in mind. I was there for about ten days in total. I’d had a meeting with Janet, I think in London, but it may have been in New York. I thought I’d have a go at writing something that would work for her and it didn’t come out at the time."
https://ivorsacademy.com/info_advice/perfect-pop-cathy-dennis-interview/
Then she says, "We did have this song Toxic, though." And 'Toxic' is the song she initially offered to Kylie Minogue, not Janet Jackson.
Don't forget, in those sessions Cathy Dennis also wrote 'Sweet Dreams of My La Ex', which sounds VERY similar to Toxic, and also not written for Janet Jackson.