"Jump sustain" just sets the vector Y to the jump strength every tick while the jump button is held, up to a time limit. That shouldn't be too hard to reimplement with events as well.
The aim of Construct is to cover the basics and then give you an "escape hatch" to customise things in more advanced ways. The Platform behavior is a great example of that: it can do a lot for you, but it also gives you control over the vector X/Y so you can do virtually anything else imaginable providing you can come up with the event logic for it.
Further, we already get far, far more feature requests than we could possibly act on, so each request needs to be strongly justified, and this appears to be already possible via the existing customisation feature. And then even if we did everything everyone asked for, there's a risk Construct could end up crammed with loads of settings to configure the minutiae of every last micro-feature and quirk, and then it's no longer easy-to-use software accessible to beginners. So I do think there needs to be a point where we draw a line and say "use the existing advanced customisation for that".