Capsium, no teacher would disagree with you. Not because they don't want to fully differentiate but because the upshot of full, 100% differentiation is that very child has 1 teacher who teaches them according to their individual needs and mores.
That is impossible. But currently teachers are being held to that sort of standard, whilst still being the only teacher in the room.
The frustration of that might be what you interpret as not caring or not expecting any achievement.
To be absolutely honest, your sweeping statements here:
Schools need to be more accountable for teaching the whole of the curriculum, especially when there are additional needs. All too often there is the attitude that because a child had needs that are even only slightly different to the norm that they cannot achieve. Often all that is needed is proper differentiation, reasonable adjustments that do not even cost very much. are incredibly insulting and just wrong.
Schools are accountable for the whole curriculum. That the curriculum is too large to fit in a school year, or even a calendar one, is not the teacher or school's fault!
The attitude regarding kids with SEN is not 'Ah well, they won't achieve anyway, fuck 'em.' That is a weird accusation to make. OK, I really went to town on the paraphrasing there, but I found your assertion offensive!
'Proper differentiation' what is that? What are the limits to your expectations, your 'reasonable adjustments'?
As for the cost... well, the in class adjustments may be cheap enough to do, but the administration, assessment, monitoring is not. There needs to be SENCO, properly trained, expert SENCO, as well as TAs and the teacher. Or any cheap adjustment the teacher makes will be based on guesswork!