Decentralisation is hard - never told the contrary.
However, transaction "gas" fees with some chains (Polygon, Solana) are less than 1 cent and you probably know that.
You can pay as much with DynamoDB if you don't optimise anything (and reads are free with the chains, still costly with DynamoDB). Of course, you can say that AWS is overpriced too... maybe that's just inflation going on.
1 cent per write to a database is insanely expensive, and no, you can't pay that much for DynamoDB. DynamoDB is free tier for 10 writes/read per second.
If you go with AWS's most expensive option for access (DynamoDB on-demand, infrequent access), the pricing is $1.56 per million write request units. That's 0.000156 cents per write. That doesn't include data storage, but that ranges between 10 and 25 cents per month, per GB.
This is pretty much just the same thing we do today, but wait...
> possibly hosted on decentralised platforms (IPFS)
We can use a filesystem that is extremely slow, and also loses all our files if we stop paying AWS to mirror them in S3!
> the backend is a decentralised blockchain
And we can use a really slow database where each write costs $100s in gas fees!
... see, much better!