If you're going to have that many spell levels, you might as well just have spell points instead. 4e had spell levels that were the same as the level you got them, but even that system didn't have 20 or 30 levels of spells, since you only got a new spell every couple levels or so and dropped lower levels ones as you went up in level. That, and you didn't have to worry about preparing most of them or deciding which spell should go in which slot for greatest effect. Having to micromanage spell preparation when spells can be prepared in different levels and having 20 levels of spells to work with would be a nightmare, IMO.