I think the main hang-up here is that the Decay Amount Total is no longer the sum of per Account. It can still be divided up on an Account Basis. Which does show a bit of an advantage to larger "Account" guilds because they have more "hands".
However, I've pointed this out at the beginning of this thread when another posted numbers like this. To fairly represent the numbers you need to consider the "Renown Adjustment".
1 150%
2 180%
3 210%
4 240%
5 270%
6 300%
7 285%
8 270%
9 255%
10 240%
Your example of 10 Account Guild with a total of 67,500 Decay a day actually requires pulling 2,812.5 Renown per day per Account. While still almost 5x per account more than a 100 Account Guild it is still 562.5 renown per account Less than a 20 Account Guild which needs 3,375.
I'm utilizing the 20 Account Adjusted Guild Size as that appears to be the point Turbine has set as a baseline.
Under the Current System with everyone having the same Decay Amount at any level. It appears that Guild Sizes of 8 - 10 Actually fair better than 20 Account Guilds, with 6 and 7 Account Guilds fairing just slightly worse on a Guild Renown to Account Ratio requirement to cover Decay.
The main sticking point is what Size should be the "Average" renown earner. I don't know what the Average Account size is per Guild on a Server. I'm pretty sure that we don't have any 1,000 Account Guilds, but I could see we may have a few in the 500 range - but I would be inclined more towards Large Guilds actually having somewhere between 250 to 300 Individual Accounts at any given time.