- Lots of salt in the sauce means you'll have very salty sauce with undersalted pasta. It might work out fine if the sauce coats really well, but will still be slightly different.
- Ingredients cook differently when salted. Not sure about pasta specifically: I _think_ I can taste a difference between pasta cooked in salted water and pasta salted after being cooked, but I'm not certain.
- Definitely don't keep pasta water around, it's just for the sauce you're currently making
This is missing the forest for the trees. The point is not to make restaurant quality pasta and sauce, it's to not have to buy salt from the store every week. It's wasteful for little benefit.
I can't ever say I've felt like cooking pasta without salt is "under salted." Is it as savory and delicious as what I'd buy at a restaurant? No - but having done it like that in the past I just think the benefits are marginal for how much you waste.
I wouldn't say it's "forest for the trees", but I see the point that it's a choice of trade-offs.
I make pasta _very_ often, I love it and want it to be as good as it possibly can, so I'm very happy optimising for taste. FWIW I use a tablespoon of salt per portion, and get a 1.3kg pack of salt every few months: hardly a huge waste!
- Ingredients cook differently when salted. Not sure about pasta specifically: I _think_ I can taste a difference between pasta cooked in salted water and pasta salted after being cooked, but I'm not certain.
- Definitely don't keep pasta water around, it's just for the sauce you're currently making