Only TCO matters, that is the efficiency you actually optimize for, ie dollar per mile[1]not miles per gallon.

If the car is going to need to be in shop for days needing you to have a replacement rental because the model is difficult to service and the cost of service itself is not cheap , that can easily outweigh any marginal mpg gain .

Similarly because it is expensive and time consuming you may likely skip service schedules , the engine will then have a reduced life, or seizes up on the road and you need an expensive tow and rebuild etc .

You are implicitly assuming none of these will change if the maintenance is more difficult , that is not the case though

This is what OP is implying when he says a part with regular maintenance schedule to be easily accessible.

[1] of which fuel is only one part , substantial yes but not the only one

I'm just gonna copy and paste a response to another similar comment:

The point that I am making (obviously, I think) is that tradeoffs exist, even if you don't think the right decision was made, your full view into the trade space is likely incomplete, or prioritizes something different than the engineers.

Putting some random number of hypothetical mpg improvement was clearly a mistake, but I assumed people here would be able to get the point I was trying to make, instead of getting riled up about the relationship (or lack thereof) of oil filters and fuel efficiency.