Look at it this way. Programs have lots of important properties. Some of these we have figured out how to encode as types so that we can verify them statically. But (although research in this area is ongoing) there are still a lot of important properties we require of our programs that can't be statically verified. The upshot is, we have to test them. Yes, testing is necessarily imperfect, but we have to do it anyway. In my experience, in the course of testing a program in a dynamic language, the kinds of errors that would have been caught by static typing are relatively easy to find by testing; usually, the errors that are hard to find by testing are also well beyond the scope of static typing. Maybe that will change eventually, though I'm skeptical, but it's certainly the state of type systems in common use at the moment.
So the parachute analogy is grossly exaggerated; it's more like leaving your shoelaces untied.
When Doel says "you're depriving yourself of the ability to state and enforce the invariant...", it's not clear whether he means "always" or "at least sometimes". I concede that he could have meant the latter, in which case you're right, my counterargument fails. But as I've just argued, there are lots of other invariants that we can't statically enforce anyway, and they tend to be the more important ones.
2) A table is a bag (multiset) of tuples of some specific type (schema). When you add a column, the table now has a different type, because it's now a bag of tuples of a different type.
Imagine that instead of using a database you keep all your data in a running process of a program written in a static language. (Let's ignore the possibility that the program or the machine might crash.) How are you going to structure the data? To make use of static typing, you need to use collections of records (or "structs" or whatever you like to call them). How then would you add a field to one of these record types? There's no way to do it without recompiling your program, and there's no way to do that without either losing your data or dumping them to files and reading them back in to the new version of the program.
Now, you could store your data differently, by explicitly modelling rows as maps from keys to values. But what's the range type of your map? Why, it's the union of the types of the possible data values you might want to store; you've lost static typing for the data values, and will have to check at runtime whether each value you read out of one of these row maps is of the correct type.
