Here are some of the problems I think might occur from this Social to Military system.
1. Inability to tell what production is needed for your ships. You'll have to add Social and Military Production together. Only you wont know what the Social base number is, so it would be useless without a calculator by your side to do the math.
If Planet X has 70 Military, and 30 Social, you will be able to tell that 70 goes into 140 two times. So you could build a 140 Colony Ship in 2 turns. Thats the Current system.
If Planet X has 70 Military, and 30 Social, you will be unable to tell what Social number transfers over from 30. You won't get all 30 Social because much of it will be Bonuses that don't transfer over. So you would have to bring a Calculator, find the Bonuses for Social, minus the Bonuses from the 30 Social, find out its say 20 Base, now you take that 20 and add it to 70 Military. Now you find out that Planet X has about 90 Military, only you had to spend 5 minutes figuring it out. Now you can go create a ship that is about 180 Cost and build it within 2 turns. You wouldn't be able to figure things out as easily using the Social to Military system.
2. No bonuses applied to your Socially transferred to Military production. So changing the slider to full blast Military and nothing in Social would still be the most affective means to reduce waste.
I covered some of why this is a problem is #1
3. Changing sliders would have much more fluctuations than they are now. If I had Social at 50 and Military at 10, and I change it to 25 and 35, the fluctuations be too large. Right now you can look at the money being spent and say to yourself. Ok at 287 spending my two Manufacturing planets are producting Colony ships in 2 turns each with a little waste. You decide you want to trim this pork alittle. So you reduce your military alittle, and now you click the slider one down and all the the sudden all your ships built everywhere are not producing in 2 turns but in 3 or 4 turns because the bonus never applies to the Social, so increasing it adds to the ship but in turn isn't as efficient. So you wonder how much money is now being spent on Military or Social, and now you can't figure it out because Military is still stuck at 287 but your ships aren't producing at 2 turns now. And you wonder why that is, well its because your spending the same BASE amount on the ships, but the Bonuses aren't being applied to the social that was transfered over to military. SO you wouldn't be able to ever tell from looking at money being spent by your empire on rather or not your efficient as you can do right now.
So lets say they don't change add the Social to the Military and keeps it seperate on the Domestic Stats: Expenses- So now you would see your spending 280 something on Military and your spending more on Social, but you wont REALLY know how much money is going to Military because a good quarter or so of your Social is being transfered over to Military without you seeing it. You would be unable to tell how much is going over there.
So in the end, from all the frustration you resort to the 100% military, 0% Social slider. And then you just focus social when you have buildings, and take focus off when you don't have buildings.
4. The waste for overspending on ships or buildings is not refunded. As someone said there would be huge shifts in how the treasury worked. So you won't get refunded on your purchases, if I have a 200+ Industrial Giant producing 100 Waste in Social, and I build a Ship that is 200 then that 100 Social transfered over to Military is STILL wasted. Its just wasted another way. Its just musical chairs. The waste goes to another chair to be wasted. ONTOP of this waste, you wont be able to tell how fast you can produce your ships, what the accurate Military production is because you have to calculate social base and minus it from the Military to find out what the numbers are. It just makes things a heck of alot more confusing and frustrating.
5. No Social Spending if no Building. Money shouldn't be spent on Social if a building isn't in the Queue. Donald Trump probably doesn't pay the boggy man for building nothing. It makes good business practice. If money from the Treasury is never spent on Social, without a Building in the Queue, just like it is never spent on Military, without a ship in the Queue, then the Treasury would not fluctuate at all. The only arguement is that you would waste alot less which might give you an added advantage over the AI.
6. KISS - the Social to Military System is not simple, not taking funds from the Treasury with no Ship in the Queue is simple. And it can be applied to the Social System aswell.