i agree, the problem is really big, simply because u dont need to focus on economy/trade to get loads of infite troops..
i can tell a good example, i played a multiplayer with custom nations and random world:
i was a pseudo-Netherlands hardly focused on trade and i managed to get a 250 gold total income around 1600-1700 exatly to get infinite mercenaries pool but with less military ideas, of course my basic forcelimit was a bit low and i had no manpower/good generals, but my main focus was to keep wars longer to win with mercs spam against lower economies IF i coud let the war last enough;
well one player playing a pseudo-France took full military ideas (with obviusly economy-religion for military policies) and managed to keep AT to 80-90 so he had good forcelimit/ manpower, good morale/discipline and permanent 3stars generals, so it was ok that my army will lose almost every battle but you think "i'm able to spam mercs and wait his manpower depletion if i can hold enough my forts, if he spam mercs too he wont be able to sustain them as i can do with my income" well i was wrong, he simply didnt spend a CENT for 50 years, occasionally farming near OPC for money and keeping his army maintenance as lower as possible so he had a 50'000 gold reserve..
wich means nearly 4'000-5'000 REGIMENTS of mercenary infantry, thats like 5'000'000 possible manpower you can use as a full military focused country who doesnt need to build or manage trade or anything belonging to gold income,
this is not fair and is so frustrating for a player that doenst want to take always the same ideas to prevent to be steamrolled by discipline/AT stacking, so you HAVE to take military ideas as soon as you can, or you CAN'T manage to win a war, because every country can just stack gold without limit and wars will last forever.
There is another bad example on the same game: the "pseudo-France" player was finally almost totally occupated during a war because his troops were elsewere, well he still managed to mantain his 250k mercs army thanks to the occupied provinces he had in the balkans, WE was not a big problem because he had innovative, a custom national idea and a good king, he simply had that 50k gold reserve and keept spamming troops even if his country's economy was totally down and 90% under enemy control...
gold now works like manpower so you NEED a limit on maximum amount of gold you can stack like manpower has maybe depending on your adm development