No, it's not absolutely critical. You can do a lot of your own programming without using hooks and forks. Your programs will be somewhat simpler - and whenever you need to use a variable in several places, you'll have to explicitly name it - but still.

Incomprehensibility happens in part because all ASCII characters are used - so you have a lot of differently-looking symbols; because [ and { aren't paired with ] and }, and " isn't paired either; because you often have . and : as the second character of built-in entities, so you've got a lot of dots... APL used the symbols invented explicitly for those purposes, but the price - non-ASCII alphabet - was considered too high. So - no, I wouldn't say forks and hooks are the source of 90% of uncomprehensibility.