let suppose A ⊆ B means A is reducible to B .
we know every language is subset of ∑* , which is regular
so every language in this world is reducible to regular language , so ultimately every language would become decidable in that way. Undecidability gone...
A ⊆ B only means A is subset of B , nothing else...
A ⊆p B means A is poly time reducible to B , means if A is undecidable then B will too...