Unfortunately, no matter how low of a monthly fee you suggest you will never convince the contingent of users who refuse to accept "renting" software. You see, they believe that they "owned" the software when they bought in the past and could use it forever and ever and ever to infinity and beyond. They feel that "Adobe owns you" once you sign up to a subscription model because if you ever stop paying you can't access your old PSD files. And they also fear that once everyone accepts the subscription model Adobe will jack up the prices on us.
NO - what I propose if you lay out $700 for PS, you own a perpetual license. You pay a monthly support fee rather than an annual or 18 month upgrade fee. You can stop paying and keep using the s/w just to be back on the support service you need to pay missed months (either in full if short term or at discount if longer).
YES - all the complainers already own CS5/6 and don't want to pay a single dime on a monthly basis until such time as Adobe releases a new version that has new features/improvements that they deem worthy of their dollahs. Your suggestion is a noble attempt at finding a middle ground but it still means that one has to keep paying monthly to get bug fixes, updates to Camera Raw for new bodies, etc.
And it wouldn't work for Adobe anyway because everyone would just suspend their monthly account once they felt they had a version that was updated and "working for them". They wouldn't turn it back on for 12-18-24-36 months when they needed an update. Under your proposal, the fee they would pay to "catch up" would be less than what Adobe was charging for full version upgrades (for example $10/month x 18 months x 80% = $144 < $199) so Adobe is worse off.