Please answer this question: Why is mixing levels and experience points with action any more advanced than pure, well-done, detailed, thought-provoking action gameplay? Action RPGs have been around since 1984 (see Dragon Slayer on the PC88), so why do people act like the Castlevanias that mix those kinds of elements with tepid, poorly-developed action are the fucking future? If I want blase level design, boring enemy patterns, and OH MY GOD I CAN LEVEL UP I'll just go play that turd Ys III for fuck's sake. That's the main problem with Metroidvanias: they're broken from the beginning because they're designed around gaining experience to beat enemies instead of providing a consistent and engaging action-based challenge. If the three on GBA had half the ingenuity of design that Rondo has, you wouldn't have any complaints from me.
I think the free-roaming gameplay and the experience levels need to be separated in this discussion. They are not one and the same, and they do not need to be grouped together. Metroid has free-roaming gameplay, but it also has great action mechanics and great enemy patterns, and it does not allow you to unbalance the game during the course of normal play (unless you count the enemy generators, but that's minor and is a different story). Metroidvanias do, and that's why they're broken. The games require backtracking through the castle and fighting the same enemies over and over, which always results in the player becoming overpowered. Sure, you can rush through the game or impose artificial restrictions on yourself so that the game keeps some semblance of challenge, but seriously...if that's required of the player, there's something wrong with the game.
To sum up: Metroidvanias drag down the series by diluting the play experience with shoddy action-mechanics and level design, and show laziness in their overall design with how too much reliance is placed on level-grinding instead of the series' traditionally strong action. The series needs a kick in the pants with regards to all aspects of design, and experience points and equipment are NOT a cure-all.