They could, but SPVproof is conceptually more secure & decentralized.
but at more risk from unknown side effects and conflict of interest.
Server centralization is a very real risk.
Unknown side effects are exactly that, unknown.
As for the conflicts of interest, they might exist but I really don't believe and foresee how they could be used to effectively damage Bitcoin in the long run. I'm sorry but I think your theory of core devs negatively influencing the development of Bitcoin in the favor of Blockstream is bonkers.
How is the federated model so centralized? Couldn't you use a set of semi-trusted oracles like Gavin has described. This model may not be completely decentralized, but may be decentralized enough.
The answer is "yes," if we're willing to replace "verified by the entire network" with "verified by a set of semi-trusted 'oracles'."
That's cheating, though, isn't it? We're not entirely decentralized if we are trusting eleven contract-verifying-services not to collude (or all get hacked) to violate conditions encoded in some contract(s).
It is cheating a bit... but all of the really interesting complex contracts I can think of require data from outside the blockchain. Like the BTC/USD exchange rate on some future date (for blockchain-enforced futures contracts).