Well, it seems unlikely this would last forever unchanged, so the question is how English kings after Edward handle it (or Welsh princes after Llewellyn, for that matter)
Yeah, this is a good question. Well I wouldn't think the English kings would have much to do with it on their own. Even with Wales divided as it was they had plenty of trouble and as I said before, relied on the Welsh to defeat the Welsh, just like how they relied on the Welsh to defeat the Irish, the Scots, and the French.

But anyways, the big problem the Welsh had was that their ancient laws dictated that inherited land is divided equally between all of a lord's sons, and so this caused a lot of problems. Llywelyn ap Iorwerth (the Great) recognized this and tried to do things the English way by having his son Dafydd inherit everything, but Dafydd's brother Gruffydd didn't like that at all and so joined the English against him and undid everything his father built. Llywelyn ap Gruffydd got a lot of land back though, maybe even more, and I don't think he'd have had the same problem and by the time he passes on the Welsh just might be more used to doing inheritance the English way.