Thats not taking into account the higher max tech levels of the demo leading to higher income, better weapons tech and higher gains due to milstrat

Might as well play a demo from the start and get the same land in 3 grab (for similar jets) as a monarchy would in 4 grabs. Then you only lose 1 turn per day from grabs but gain in tech effectiveness and zero market commissions

And if you take spying into account - making one more attack would require at least one more spy op, maybe 2

In that case the demo actually comes out saving turns, no market commissions, higher income, higher effective military strength!