Upgrading isn't the reason to play the game 
The difference of rank 5-9 should be different anyway.
8-9 aka 100% should take the same time like to lvl your rookie lvl from Green C to B (which is a long time).
Otherwise a system with a fail protection up to 30% (1% bonus for every fail) +3% of the reputation system include.
your idea could be great but thing is if you just tell 100% xp = +1 then the grinding / farm cap is far lower and all the interest in the game is lost, because honestly if you reach instantly +8/+9 then where is the fun ? Just pass to the next mmo ?
First, this is an erroneous assumption.
In any particular MMO, there are various segments of demographics to consider with regards to monetization;
Grinders
These individuals would enjoy grinding content over and over again, and keeping them engaged entails giving them more things to do. There has to be some variability in gameplay, some new skills, some new thing with some perceived objective reward.
Give these players more rewards, more content and things to do that entail grinding; offer premium specials to unlock custom content after they have invested a sufficient amount of time. Offer premium quests or things that can multiply your grinding rewards, like EXP boosters for free, mixed with some paid things.
Expert Players
These players are highly skilled players and like difficult and challenging content. Give them more modes to explore, change the dungeon mechanics, AI of mobs, etc. Having something like a dungeon-crafting system for users with daily rewards would greatly help player retention of this type of playerbase; monetization entails having the ability to unlock harder content in exchange for money, and being able to customize and fine-tune the parameters of such difficulty.
Casual Players & Social Players
These players thrive in socializing with others and doing communal things. Group dance synchronization, game design mechanics revolving around sharing in-game experiences, contests, leaderboards for most 'X' user, matchmaking, clearing something together with a large team, etc should be mostly incorporated into the game. Monetization entails the purchase of special accessories, or access/improvements to specific sub-systems of the game which enable expressivity (group emotes, recording mode, marriage system, fun events q/a rooms, specific 'group' actions that have in-game FX effects).
Achievement-Orientated Players & Status-Orientated Players
These players seek the need to get achievements, and the first in everything. Simply adding leaderboards for fast clear times, first cleared, first to do something and any other possible mechanic with artificial limits to being the only one to have 'X' or 'Y' will facilitate this sort of competitive environment for such a playerbase. Monetization entails successfully guiding a clear goal-orientated path of gameplay with defined outcomes with server announcements / relinquishment in show of competency.
For status-orientated players, artificially limit the number of items / titles you can get something and have a defined set of things they must do in combination. Some can involve spending, some may not. Improving an avatar's social status through a combination of F2P and P2P means is probably the best.
Thrill-Seeking Players
These player seek the thrill and action of fast-based combat; death invasion wave-style dungeons with increasing successive number of mobs/difficulty, and other sorts of various activities that are engaging involving full attention like raiding probably caters to this group the most. Monetization in the form of fast gear track progression and skipping the grind probably works the best here.
Reaching instant +8 or +9 itself is not equally valuable to everyone, some enjoy the journey to upgrading the weapon, but not everyone gets to have this opportunity. Obviously, you don't want to make the game Pay2Leave but if this game had more content and things to do, paying to skip the tedious parts and do the things that people want to do is a perfectly reasonable method of monetizing the game. Furthermore, if this system were implemented, it would be highly likely to be difficult to +8 or +9 your weapon without spending 45-60 days of hardcore grinding, meaning those people who value playing and grinding more than their time could do so, while being able to spend on other aspects of the game. There is no perfect one-sized fit all solution for MMOs.
Nearly every complaint on forums can be traced back to 'economic rationale' reasons - poor returns, poor systems misaligned and ill-equipped to appeal to a broad playerbase, garbage and inferior monetization schemes to other versions.
Why do complicated system when tou can just change the numbers of sucess rate easly ?
And gameforge will never do that.
