Why is it assumed we need to implement fully decentralized smart platforms in the Bitcoin system?
Because Bitcoin is the one & only trusted ledger. If it isn't built within Bitcoin then you sacrifice trust.
Blockstream said it best :
Can't be evil Bitcoin & friends don't operate on trust, they use proof of work. Have you even read the whitepaper?
And Bitcoin is far from being "the one & only trusted ledger." Litecoin, Peercoin, and Primecoin have been doing very well for a couple of years, TYVM. Why would you only trust only one form of proof of work? If a SHA256 or ECDSA exploit is found, BTC goes to zero.
Platforms are value neutral. TCP/IP is being used for evil as MS/Google/Apple/Facebook aid and abet the creation of databases that would make Stalin and Honecker blush with embarrassment and shame.