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.
1. If both SHA256 and ECDSA are cracked then you still can't crack cold wallets with no outbound transactions.
2. If one would crack Bitcoin they would be stupid to drive the price to zero. You don't kill the goose that lays golden eggs.
3. Who says Bitcoin can't change it's proof of work algorithm?