Yes, YABU.
These shops sell different products and a lot of them are good quality. Yes, Sainsburys has bigger variety so I won’t be able to shop only at Aldi and Lidl but it’s very much depends on what you are eating/ cooking.
Potatoes, carrot, broccoli, tenderstem broccoli, leek, courgettes, mushrooms, tomatoes, peppers and onions ( red, white and spring) are absolutely fine and cheaper in Aldi and Lidl. The same apples to bagged salads, apples, avocados, lemons, limes, pears, blueberries, raspberries and strawberries. Although over summer Sainsburys sells the best strawberries. Kiwis are hit and miss, but the best are in M&S. Other fruits and vegs are absolutely ok. Milk, eggs, cream, unsalted butter, flour and caster sugar, Parma ham, chicken slices, whole chicken, chicken mini fillets, chicken drumsticks, diced beef, fresh salmon and cod, sea bass, smoked salmon, cooked prawns, falafels, vegetarian sausages, kitchen towels, toilet paper and even ice cream - all of these are absolutely fine.
I agree about variety. The choice of yogurts are not enough for me, and I do like a particular brand of cheddar cheese, not from Aldi, but Brie, feta and goats cheese from Aldi are really good. Aldi has much less spices, bakery is good but again less choice, but all the main products are there. I don’t like their jams, tea and coffee but oils are fine. And Lidl has very good choice of chocolates, and I’m chocoholics with huge experience. Of course, if you are into organic stuff, agree, Sainsbury is the best place.