Yes it's very fast but still limited in term of transactions per second (even if it's way better than Bitcoin). If it becomes very very successful, the hard limit might be reached.
Good point, but lightning means more centralization and less trust, doesn't it? An issue here is whether the community will take seriously such "improvement".
But you can still use the main chain if you prefer. The most useful would be for example if institution A does a lot of business with institution B, instead of spamming the main chain with thousands and thousands of operations, they can use a channel between them (less fees, no delays, privacy).