Mistwell
Crusty Old Meatwad
Exactly. Why assume a reason that WotC would have stated and they didn't? Totally illogical. We have eliminated the possibilities and are left with killing the competition of old versions...
No, it is illogical to assume your answer is the only possible answer.
1) In no interview that I know of has WOTC been directly asked about why they discontinued the non-4e PDFs.
2) Hence, to assume they have not stated a reason on purpose is not logical. They have not stated much, beyond what they were directly asked. There is no real reason for them to have stated a reason for this specific issue so far, particularly if the assumption that they were not selling a whole lot of them anyway is true.
I do think it's safer to assume they do not care, more than it is to assume some nefarious conspiracy theory regarding competition.
Also, you have definitely not eliminated the possibility that they do not care, or that they were not profitable. THAT is an illogical statement. The burden on you to entirely eliminate other possibilities is a heck of a lot higher than you simply saying so.