[rodneyzeng]

The normal approach to prove Thale's theorem should be induced from the property of central angle being twice of an inscribed angle that subtends the same arc. Since a diameter has central angle of 180 degrees, its corresponding inscribed angle should be half of 180, that is 90 degrees.

[lupire]

No, because if you do it that way, you wouldn't have Thales's Theorem. It would be Thales's Trivial Corollary.

Thales's Theroem is a simpler, easier to prove (as in OP), less powerful statement than the inscribed angle theorem.

[rodneyzeng]

Your second sentence denies the first sentence. The proof of the Inscribed angle theorem does not need Thale's Theorem, and it is stronger than Thale's Theorem.