The first example would see no slowdown in production at all, as you have on hand the minerals to make all of that stuff anyways, plus however much more you get per month until the projects finish (the 500 minerals a month still accumulate, just are paid against the negative payment for the project)
The second case what the monthly income would cover some projects and it would continue down the line as they finish like any other queue system, top gets priority and when demand is filled excess goes to the next until no excess is left.
And furthermore, for the second case, it requires you have discipline. Which you need when you're spending from the bucket, you prioritize what you need/want and get to the rest when the bucket fills up.
The production screen also would make utilizing sector based production lines more easily than finding and queueing that way, or setting them to hotkeyed positions which are limited.
All in all I don't see the fuss over it, a they have a proprietary precedent of this in their other IPs, though they utilized other resources. I'm sure switching to one only(ie removing oil energy etc from Hoi series and using minerals) and also starts to address some other parts of the game (wars are effectively over after the doomstacks meet, excessive mineral bloat for large empires now the monthly income funnels to their production)
And seriously, more complexity?
Build x, y, Z and watch your EC income and make sure it says +1. Ie, players already evaluate a resource and it's consumption on the fly, which will also become minerals(and sooner or later, food)
