Currently, their whole business model is based on donations I guess, so you're free to donate.
But I'm concerned about this as well. Construct is becoming more and more powerful and we all want to set up a working pipeline and then just stick with the tools, be creative, get the most out of them. If the tool isn't being supported in the future because the devs have other gigs that keep em busy, we're all in deep shit. If Adobe would decide that they'd cancel Photoshop, I'd head over there to nutkick their management as well.
Just because an app is free doesn't mean that you haven't commited yourself to it. Just because you don't charge money as a developer doesn't mean that you're free of responsibility.
So, personally, I wouldn't have any problem paying for Construct. It's a quality product already and I'll use it to create a commercial product, so it'd only be fair if the developers would get their share.
Since Construct started as a free project though, a lot of the users here would not have the money to pay for it. So, you'd have to be clever about setting up a business model.
A viable option could be something like:
Construct stays free and you can still simply download a full-featured version, but you can't compile to any platform with the free version. You could test the runtime just fine, but you can't compile it. If you buy a license, you'd get a couple of new buttons that'd allow you to compile for the PC, Mac, X360, etc. Heck, I'd drop a couple hundred bucks on it if I could deploy to more platforms. It'd be a win/win.
I guess a business model like that could be attractive for most users. You could still learn the app and only if you're going to deploy your game, you'll have to pay for it - it's a fair deal.