This [1] article says the current discount prevents witness spam. Witness spam is free space in the witness part of the block that can be filled by miners to create bigger blocks with almost no cost for the benefit a cluster of miners with low latency, increasing centralization.
The 75% discount does not prevent it, but on the contrary leaves a lot of extra witness space for spam.
If the maximum block weight is set to 2.7M, each byte of non-witness block costs 1.7, and each byte of witness costs 1, then a normal filled block would be 2.7M bytes (1.7+1), and there will be no need to create ever a 4 Mbyte block. The worst case would be the average case, and the transaction rate would be the maximum possible.
The current 75% discount can only achieve more transactions per second if the type of transactions change. Therefore the current 75% discount only makes the block size worst case worse (4 Mbytes when it should be 2.7 Mbytes).
80% of all inputs/outputs are P2PKH. The only way to make use of the extra witness
space If most P2PKH transactions are replaced by multisigs (typically for LN).
So it seems the 75% discount has been chosen with the idea that in the future the current transaction pattern will shift towards multisigs. This is not a bad idea, as it's the only direction Bitcoin can scale without a HF.
But it's a bad idea if we end up doing, for example, a 2X blocksize increase HF in the future. In that case it's much better to use a 50% witness discount, and do not make scaling risky by making the worse case block size 8 Mbytes, when it could have been 2*2.7=5.4 Mbytes.
I've uploaded the code here: