just having a blockchain transaction is safer than having a unconfirmed privately exchanged smart contract.
The point that you don't seem to want to understand is that you can't make a comparison like this with such a broad spectrum. For example you don't know how the private smart contract on a centralized platform is being enforced and you also can't know the security of the public blockchain is.
Such things have to be compared on case by case basis.
cypherpunks 1998-2008 were looking at "smart contracts" in regards to 'p2p money' and kept running into the issue of who gets to audit/arbitrate/settle in situations of disagreement, dishonesty, blackmail, extortion.
smart contracts without blockchains can work in times of utopia/mutual agreement/co-operation. but the world is not perfect. which is why courts, auditors, arbitrators are used. and blockchains solves that by having an audit system that cant be corrupt as easily as a smart contract alone.
yes a blockchain of just 1cpu hash power is not as strong as a blockchain of 1.5million ASICS.. but in times of dishonesty. a blockchain of 1CPU is 1 step removed from the problem. because to defraud a blockchain require more then just deleting a file. it include having to do some work to replace someone elses copy.
yes PoS blockchains are less secure then 1.5millions asics. but a little more secure then 1cpu PoW . but blockchains as a whole is one extra step that prevents a smart contract from being simply deleted from existance.
oh and also
we all know that a centralised database or even a centralised blockchain is a single point of failure. where as even a cpu mined and 5 archive node public blockchain is more secure.
we can even go further.
where there is a network of independent peers. that have created their own unit of measure[token] that has no blockchain to settle against. by itself. is unsecure compared to settled blockchain coins. no matter what the blockchain hash mechanism strength is
we can go even further then that.
even if all users of a network of independent peers. all got a copy of everyones smart contract. but NONE of these were timestamped into a locked system which can categorically identify any edits/deletions. (by use of comparing hashes). is less secure then a ledger that is locking transactions into a block with a hash that can be used to test the block for edits
do you now see why the cypherpunks actually got excited and interested in blockchains invention.
when a guy in late 2008 announced an idea, and within a few months had a working software to show it working. it completely solved a problem cypherpunks were trying to scratch their head about for 10 years in regards to p2p money