Reasons to sub:
Scirra supported exporters
Improved workflow/editor elements
Runtime update, when it happen - This will probably be the biggest reason to use C3 over C2, when they get implemented
Able to work on mobile/linux/mac
If these are of no use to you, then purchase C2, which is and will remain a one off payment, or use other software, of which there are plenty of alternatives. If you have checked out the alternatives and are still here complaining about the price, then you are simply justifying the fact that Scirra has something worth paying for.
Regardless of how you want to define "locked out", it does suck that you will not be able to edit your C3 projects after your subscription expires, with no way to export to C2 besides manually recreating the project. To update a project after a subscription expires would require another year. Monthly access, perhaps at a higher price rate, could alleviate this. Or it could only be for previous subscribers if the worry is that people will create projects in C2 to export in C3 (which doesn't seem like too big a deal to me if they did buy C2 after all, which is more or less equivilant to a year subscription in C3). I imagine monthly price would be in the $20+ range, which would probably kick off another round of complaining. It shouldn't be too difficult to implement though.