Seems like both ways achieve the same goal, only some like one and others like the other.
Totally fair call-out. I'll endeavor to describe my perspective and why I'm pushing for this. I'll note up front that I get caught up in these sorts of design discussions because I want to make things 'better' from my point of view, and help. If Joseph comes on in ten minutes and describes why he knows he absolutely wants a spell level system I'll just drop this and flow with it. The last thing I want to do is push anything towards a designed by committee type outcome.
I'll have to ponder the ramifications of your system, but I should point out that what you've done in essence is to divide spells into those which are simple (corruption 3 or less) versus those which are complex (corruption 8 or less). This is in effect a spell level system whether you want to call it that or not.
I'm not against levels of power, and the intended implication is that a system something like this would allow fine gradations, so that low threshold might be 4, 5, 6, etc. or even higher.
So, let me take a step back and give my larger wizard perspective.
As written right now Wizards get a majority of their spells at first level, particularly if they have any intelligence bonus. These spells are randomly selected from a list and off you go. The magic system implies danger associated with magic, but the selection is binary. Either you cast a spell and thus risk the danger, or you cast no spell.
I'd like to emphasize two aspects of the game and adventure world. One is to make the quest for arcane secrets a larger and more important portion of the Wizard's life. The second is to make the dangers of magic or more active decision. By making all spells potentially accessible to Wizards at any level you say "go find this spell, and you can attempt to cast it, if you are crazy enough to do so, but the risks are extreme."
So, I'd like to see a system where arcane spells are designed by complexity/power, but not explicitly linked to class level. I'd also like to see lower level Wizards get one or two fewer spells, and the capability to gain a few more spells than currently listed as you go up in level*. I could see a system where there is a short starting list of wizard spells to roll against, but the overall list of available spells is available for learning/use when you go up in level, if you have found the proper grimoire. And I dearly like the notion that a low power wizard can attempt very powerful magic at high risk.
Spellburn may need re-visiting in what I'm proposing, but I'm not convinced that it doesn't already. I think having a limit of spellburn per spell based on character level might be appropriate.
A missing piece in all of this is exactly what roll making and reading scrolls is intended to have in the game. Is reading a scroll just like casting a spell you don't otherwise know? (I.e. you just make a spell check) or does the writer of the scroll make the spell check when recording it? The thief description implies the former. Can you read scrolls of higher level spells than you can learn in the current system? Is there any penalty for attempting to do so beyond normal casting of a spell?
* Two factors within the current system could potentially address my thoughts.
One is that it could be possible for Wizards to copy grimoires to make scrolls of spells that they do not "know", either through some spell or other ritual. Although this brings us back into the question of can you currently read a scroll of higher level spell than you can know, and what are the risks? And if you can, why not allow the player to "learn" that spell with the same repeated risk of casting?
The second is that perhaps there is a mechanism for "forgetting" spells and learning different ones, again I assume at the interval of going up a level.
Bottom line, I'd rather see Wizards start with a limited set of more mundane/common spells and feel genuinely driven to seek out more powerful magic, and be able to take the risk reward of dabbling in it 'before they are ready'. In traditional D&D magic-users would obtain pretty extensive spell lists. I'm just not seeing the same incentive here, both because you can only learn a few new spells in your career, and because even the starting spells are mega-powerful when you start getting very high results rolls.