Opinion: SPV undermines BTC. The first rule of SPV is you dont talk about SPV.
The blockheader is only 80bytes and so each time a block is added, the size of the file only increases by 80 bytes. This means a user can check their transaction is valid on the entire blockchain simply by downloading the blockheader file which is small.
The reason why BTC'ers try to deride Simplified Payment Verification, an entire section of the whitepaper, and try to sweep it under the carpet is because if it works, it undermines the whole principle of BTC and small blocks.
One of the fundamental reasons for having small blocks in the first place is how can the average user verify their transaction if the database is huge? They cant, so we need to keep the database size small so that everyone can check their transaction on the blockchain.
Yet the answer is and has been staring them in the face all along, youre just not allowed to talk about it.
SPV explains how to overcome the problems of big blocks, and thus completely negates the need for small blocks, by allowing a user the ability to check their transaction is valid by only checking the blockheaders.
The blockheader is only 80bytes and so each time a block is added, the size of the file only increases by 80 bytes. This is nothing.
This means a user can check their transaction is valid on the entire blockchain simply by downloading the blockheader file which is small. So you can verify your transaction with just your phone, running a lightweight software.
This provides enough security for almost all transactions. Only if you deal in high value transactions, might you want to verify the full transaction on the blockchain .
This simple mechanism undermines the entire need to have small blocks because if you can verify your transaction with just the block header file, the size of the block is not important, and can be as big as you want.
This feature threatens the entire existence of BTC's need for small blocks in 1 fell swoop and sends one of its pillars of small blocks tumbling down.
Satoshi already figured out the problem of verifying transactions if you have big blocks. Its staring us all right in the face and can only be ignored for so long.
Once the simplified payment verification is implemented, the arguments for small blocks will break apart. The market will decide this. Because the justification for small blocks disintegrates.
Added note:
As a comparison if you download the whole blockchain in BTC to check your transaction is valid, you need to download a 500GB file which grows around 60GB per year.
If you use SPV you just need to download a 50mb block header file that grows 4mb per year. So you can check your transaction is valid with just a very lightweight software on your phone.
Simples.
