Indian vegetable dishes are frequently cooked well past the point an American recipe would call for. The effects on the vegetable are mixed rather than uniformly negative.
Heat breaks down the structures holding cells together
Plant cells are bound by pectin in their walls. Sustained heat degrades that pectin, which is what turns a firm vegetable soft.
Acidity slows the breakdown, so a tomato-heavy gravy keeps vegetables firmer for longer than a plain simmer at the same heat.
Alkaline conditions speed it up dramatically, which is why a pinch of baking soda softens tough legumes and also turns green vegetables drab.
Water-soluble nutrients move rather than vanish
Vitamin C and B vitamins dissolve into cooking liquid. If the liquid is discarded, they go with it; if it becomes the sauce, they stay in the meal.
Indian cooking mostly keeps the liquid, since the vegetable is served in its gravy rather than drained. That is a structural advantage of the format.
Some of these vitamins are also degraded by heat itself, so long cooking reduces them regardless of where the liquid ends up.
Some compounds become more available
Cooking ruptures cell walls that would otherwise pass through digestion intact, which increases access to certain carotenoids and other fat-soluble compounds.
Fat in the pan assists here, since those compounds are absorbed better in its presence, and Indian vegetable dishes are rarely cooked without it.
The trade between lost water-soluble vitamins and gained fat-soluble availability is genuine, and it is why blanket statements about overcooking are unhelpful.
Texture is the deliberate goal
Many dishes want the vegetable to collapse into the sauce, carrying spice through the whole mixture rather than sitting in it as discrete pieces.
That is a design choice about how the dish eats with rice or bread, not carelessness or a failure to stop at crisp-tender.
Where crispness is wanted, the same cuisines use dry roasting and quick tempering instead, which shows the long simmer is being chosen rather than defaulted to.
Practical adjustments in an American kitchen
Adding hardy vegetables early and delicate ones late gives both textures in one dish without altering the total cooking time.
Frozen vegetables from a US supermarket are blanched before freezing, so they need less time again and turn to mush if given the full simmer.
Finishing with fresh herbs, lemon or a raw onion salad on the side restores brightness that the simmer removed, which is how the meal balances rather than the pot.