Since I have to dot all the eyes and cross all the tees: Legitimate use is anything that is not a spam attack, meaning where people use bitcoin to transfer money or in other words when Bitcoin is the "peer to peer digital cash system" that it was intended to.
If the sole purpose of bitcoin was just to transfer money what's the role of OP_pushdata ? And especially OP_pushdata4 ?
Seems that the creator had a different view than yours .
Satoshi was not a god, he has made a lot of weird decisions when implementing Bitcoin. Unless he has explicitly expressed his intentions for a decision you can not make such conclusions like the one you just made here.
In this case if I had to guess I'd say there may be two reasons for this decision:
1- Scalability of the code.
As a developer, specially when you are creating a consensus critical code for a decentralized system, it is easier to have an functionality and never use it than not having it and wanting to add it in the future. Introduction of other functionality like interpretation of SegWit outputs, adding OP_SUCCESSx, etc. are doing exactly that.
2. Translating DER lengths.
I think this is the most possible reason. Since Satoshi was aware of DER encoding (used for signatures) and DER lengths have a similar approach to encoding data lengths and also support very large values, Satoshi might have adopted the same approach and implemented as big a size as he could.
Otherwise I doubt that Satoshi ever intended to let anyone push 4+ GB data to the stack that only supports pushing data that is no bigger than 520 bytes.
The reason is pretty simple: the adoption hasn't grown enough for the current block capacity to not-be enough for the legitimate usage.
And how do you measure that? Even if we were to follow that route, and have a block size limit that corresponds to the "legitimate usage", how do we begin? In my opinion, the adoption has grown a lot since the last time the Bitcoin community had had this debate.
It has definitely grown but considering how fee spikes have never been severe or long lasting without spam attacks we can say with a high degree of certainty that the adoption has not yet surpassed the capacity.