The key point is that right now full node security means that no mining majority, not even 99.9% of the hashing power, can fool you with invalid blocks.
If the miners try to raise the block reward, or allow spends of non-existent coins, or anything else that violates the protocol, your node will reject those blocks as if they were never mined at all. As far as your node is concerned, those messages might as well have been a Rickroll video mistakenly broadcast on the Bitcoin network instead of a block no matter how much proof of work they represents.
SPV nodes (as they are currently implemented) can't do that. All they can do is some basic header checks. If a majority of the miners started giving themselves a 100 btc/block reward today, every SPV client in the world will happily follow their chain because it has the highest proof of work.
Fraud proofs are about allowing clients that do not have a complete blockchain to conclusively know that the chain with the most proof of work is invalid and thus should be ignored.
The situation model where fraud proofs are relevant and needed is the situation where a majority of hashing power is extending an invalid chain, so you can't say that fraud proofs are unnecessary because a majority of miners have agreed on something (like a UTXO set commitment).
Thanks for the explanation. Even without fraud proofs it is still possible to have a high degree of confidence in a system. Fraud proofs provide absolute cryptographic certainty, but there are other mechanisms that almost full certainty.
Let's use the UTXO commit set example, where each block contains a hash of the current UTXO set and fully validating nodes start assuming the UTXO commit set for the latest block is valid. And let's also assume that 60% of the miners started to create a false chain giving themselves 100BTC/block rewards. Yes a new node entering the system would and using the UTXO commit set would not be able to detect this. However that node does not have to.
There only needs to be a single node in the entire world that maintains the full transaction history to identify, flag and provide proof of the corruption. That single node would be able to provide cryptographic evidence to the world that the current UTXO set is invalid and which chain all nodes should reject.
As long as no entities are shouting that the current P2P chain is corrupted, you can be very confident it is secure. It the current P2P chain ever does become corrupted, it is very easy for 1 node to highlight this to the world.
Personally that is enough confidence for me. Maybe it is only 99.9999999999999999999% secure, I can live with that. The problem with the blockstream devs is they are trying to force 100% confidence into the system, and breaking it for everyone else in the process.
Edit: BTW, This method of probability based economic or market confidence is the exact same as the cryptography Bitcoin already relies on. The hashes Bitcoin relies on are NOT 100% secure, they are only 99.999999999999999% secure. Same with the private keys. They are not 100% secure, it's just the odds of someone replicating one are extremely small. But the odds are not zero.