You're right, it's not sustainable at all. And lots of teachers do compromise on quality in order to survive. Some teachers rarely look at their pupils' books, for instance, and have no idea how theyre actually doing in their subject. Some schools have feedback policies which have minimal teacher marking, and rely on general whole class feedback. There are lots of ways to 'cut corners'. But ultimately, once you've cut all the 'easy' corners that don't have much negative impact, eventually you get the point where the education of the children suffers more and more, and there are no more easy corners left to cut. You end up cutting out too much.
I'm speaking from a secondary English perspective, where I teach English to over 100 children (am I correct in assuming from your post you're primary, and have maybe one class of 30?). The pupils I teach write essays... hard for them to mark themselves. Higher up the school they don't do much in the way of worksheets - a lot of it (quite rightly) is extended writing. If we are expected to mark at one piece of written work per fortnight per pupil (which isn't unreasonable, as I'm sure many parents would agree), with say 100 pupils (which is a very small amount), that's still around 50 pieces of written work (including full essays) per week (with the majority of their work being unmarked and even unlocked at). If each piece takes just 6 minutes to mark (ambitious for GCSE and A Level pupils!), that's still 300 minutes. That's more than 5 hours straight away of marking time per week. So, say an hour a day, Monday to Friday, on top of teaching and planning time... just to give each pupil a measly 6 minutes of individual attention to their written work a fortnight. And in reality of course, many English teachers teach far more than 100 pupils. Maybe double. So then that would be 2 hours of marking a day just to give each pupil 6 minutes a fortnight. (I'm using a rather arbitrary example of 6 minutes of marking, but you could apply this to a number of things teachers should be reasonably expected to be able to do for the children they teach.)
The issue is not that teachers are making martyrs themselves, and doing work they don't need to do. I detest that sort of gaslighting actually. It's that the job is simply to big for the amount of available hours. Teachers in secondary schools in England teach too many pupils, have too many classes, and, crucially, have a poor non contact to contact ratio. It's interesting actually- here's an article if you want to know more: https://www.tes.com/magazine/analysis/general/teacher-workload-how-teaching-time-varies-around-the-world