>>INSIGHTS TO THE DAF-

Rebbi Yishmael was not saying simply that the object is Tamei if one did not guard it at all, but rather that it is Tamei even if one did guard it but thought that it was a different object.<<

Probably worth pointing out that there are meforshim on the Gemara in Shabbos where Rabbi Tarfon said 'kama gedolim divrei chachamim" and forbade reading by candle light, rather than giving a reason and allowing us to be moreh heter based on the reason. I believe this is similar to what you're sjuggesting.

Gershon