I don't understand it either.
A free version doesn't cost Scirra much, but brings in lots of value, even free users help Scirra big time.
Free users make the community bigger, a big community in itself is a valuable "feature" people look for when picking an engine, a bigger community creates more advertising for Construct, not only because it's bigger now, but because people can actually fully use it ...you could make a YouTube tutorial video about Construct 3 right now, and people can use it, but you always have to stay in the limited test version restrictions if you want to actually share it with the audience, so it results in only shallow tutorials because more in depth videos don't get many views. You can't say, hey let's use Construct 3 in this game jam, because everyone needs a sub otherwise you constantly run into limits. Even if you make that work, it's not fun and makes the first experience with Construct3 for these people annoying. People will be more relaxed about the subscription model because now they aren't forced to pay to get rid of annoying artificial limits, now they pay to gain access to additional services. There are so many more good reasons for it like more potential addon creators, free users simply getting used to the tool and thus preferring it, increasing the chance of becoming customers, YouTubers like Gamefromscratch would actually cover Construct, etc. etc.
It obviously needs a strategy how to turn free users into paying customers, but I think Construct is already well equipped for that. There are many features that actually make sense to put behind a paywall, you should only be able to exporting to the Scirra Arcade with the free version. Remote preview should be possible in the free version but only a very limited amount of times to show what you miss out on. Maybe Scirra offers its own cloud service, and free users can only save in the cloud with up to xMB available space, paid can save where they want and get a bit of cloud storage with their sub, as well as all the export options, the cloud build and minification service. So basically charging for services and convenience.
So I'm not advocating for an (mostly) unrestricted free version because I don't want to pay, I advocate for it because I don't want Construct to lose against GDevelop, I don't want to be the next Clickteam Fusion in a couple years.
But I obviously can only judge this from the outside, I genuinely think it would be the right move, but I recognize that I don't have access to any data about subscriber counts, if they go up or down, Scirra's financial situation, their burn rate etc. and speculating about the right strategy is easy when it's not your company on the line.