Yr argument sounds logical but just because you are below FL100 and 250kts doesn't necessarily make you more easy to see. Grey/green @ 250kts rather than 540kts against the backdrop of UK foliage then maybe you are right but white/silver/747 at 250 kts vs 280kts, then it isn't going to make much diffrence.
That is why I tend to favour the noise agrument - however, I have not seen it written anywhere.
So why for civ jets and not for mil? There is no way military FJs should have the limitation of 250/FL100 - 'kin pointless and poor training! So one wonders why it is there at all in CAS?
As an aside, it is arguable that TCAS degrades old-fashioned 'lookout' as people trust the wiggly amps so much more in Class A-D but then, they are lulled into a false sense of security in F-G.