I find the largest issue with the static GDP is that when you max it’s value out it cannot grow anymore and neither can the values associated with it (Taxes).
in the midgame once I finally get the country out of its funk of crime/poor economy etc. I get the GDP growing and doing so rather quickly, the tax rate is at ~25-30% and every cycle I can afford to add $10-30Bn of new policies to the budget .
this continues for a bit.
then the GDP maxes out, growth stops and that all important tax growth Which I am using to fund my Technological Socialist paradise comes to screeching halt. education is high, efficiency is high, production is high and I have enough inputs (especially after all of the policies really get going) to legitimately have 130-170% GDP but my tax credits stay the same and the education -> industry&tech -> GDP -> health&wellfare pipeline comes to a screeching halt and I have to raise funds through other means instead of by taking advantage of my superior production.
the other effect of this is that external conditions are meaningless. the global economy crashes? no effect on GDP or tax credits. my minister has been shagging the secretary? no effect. nobel prize? you guessed it, no effect.
as a follow on this also kills any further negeitive effects of rising GDP/industrialization because since the GDP is static no increased polution/immigration/alcoholism so on and so forth.
it would be nice to have the game be able to model society reaching a point where the amount of free cash starts to get noticably detrimental to society.