Existing ideas aren't deleted - they will just be closed and tagged with something like "Archived 2024", so you can still see them all, but they won't be under active consideration any more.
There are all sorts of ways we could organise this and I don't think there is any one perfect way. Sure, we could auto-close suggestions without enough votes, but there is now unlimited voting, so everyone could just vote everything up enough so nothing much ever gets closed, and then the system has failed to keep things fresh again. It's also possible something gets highly voted, but then sits there for several years, becomes redundant for other reasons, but then never gets closed because it remains above the threshold.
The main thing I want to avoid is the feature suggestion list building up with 10+ years of work with such a huge list of individual suggestions that it's too much time for us to even review or administrate it all properly. Over long periods of time it then also ends up full of all sorts of older ideas that are redundant or no longer important for other reasons (e.g. at one point "make an OUYA exporter" would have been popular - now it's irrelevant). Then from our point of view it becomes something of a millstone around our neck: it's such an impossible amount of work it's stressful to even prioritise or consider it, and people start to get more negative about it ("this has had X votes for Y years, why haven't you done it already?", "look at Scirra ignoring the community", etc.)
So by that point it's no longer an effective system. That's pretty much what happened before, and I'd rather avoid anything that might end up the same way. Closing everything and starting afresh every year might be annoying for some, but submitting an idea you care about once a year isn't a huge amount of work. If you have dozens of ideas we probably can't do them all anyway as that ends up creating an infeasible amount of work for us, so we want people to think carefully about their requests, and only submit the most important ones, not flood us with impossible amounts of work. If something remains important and popular, then I expect it will keep coming back every year and being voted up again, and we'll take notice of that.