Well, the market will still be the same regardless if you lower the price from Daybreak, the price will lower on Krono from 3rd party sellers.
Initially the first 6-9 months you might see a spike in sales for Legit subs and Daybreak bought krono, but it will settle back down to the same situation - Daybreak Krono for 100% profit, 3rd party 50-66% of Daybreak value. Whatever you set the value too.
So essentially, spike in volume for Daybreak first few months till it floods the market, then market settles and you are back to the same situation. What will change is there be increased volume of sales from both Daybreak and 3rd party sellers, compared to what the situation is now. The more affordable it gets people will find more value, and thus buy in more, thus volume increases, but the market equilibrium will remain same so volume of legit subs and volume of 3rd party sellers selling subs will settle to the same level over time (6-9 months is my guess).
But to your question, would I buy a sub at $9. No. Because I'm paying $0 currently vs $9. Their customer service and lack of development of the game is already pretty bad, I'm not sure at $9 a month if its value to me still. If its at maybe $3 a month with their shit customer service and development of the game, maybe.
Same thing with the $72 sub plan last year, I didn't buy into it because I pay $0 per sub since I get all my Krono indirectly from Daybreak. Someone else is paying Daybreak off.
Even if you buy a Krono from a 3rd party retailer you still win over the $72 year plan even, unless you are actually going to utilize that many Krono every month of the year. Obviously, not everyone plays like that, some people play all months of the year, it really depends on peoples play styles if its worth it to you.
On a slightly different topic, their business model is completely wrong for todays time, with so much competition out there. This is also why I think Pantheon -RotF won't succeed either with a sub plan model.
What EQ1 should have done years ago is made the game completely free (except expansions/DLCs) and fixed the core of the game, i.e graphics, avatar models, better UI. This would have brought in "new" players and a 2ndary effect of loyalty to buy into the game more often for current players thus lowering the number of returning players and people being more loyal to the game.
Imagine if they had 20 visible slots for your avatar you could buy fluff for, then there is pet fluff, mercenary fluff, mount fluff, etc. Potential is so huge if they fixed alot of the core of the game, but their to stupid and blind to see that.
The marketplace fluff is where they get majority of their business already (including Krono sold), not from regular subs.
You only have to look at ESO to see how well their model is succeeding, the only thing you buy is the DLC's.