I have realized my own real problem with Australia and Canada, and why do I dislike them being in game, is not even with modernity per se but rather the fact they are so culturally similar to other civs already present in the game. We have England, America, Australia and Canada - four Anglosaxon civilizations which emerged as powers in the industrial or postindustrial era.
Four English - speaking civs but only one civ for India, China, Arabia, Persia, Hispanic world, Germany (disunited before 1870) etc. That's so disproportional (10% of all civs!) And... Boring. At least Mexico would be something much more culturally unique and flavourful, as it is much more different from Spanish culture than Canada is from English, not to mention it existed for much longer time. Bolivar's state was very short lived but it is interesting embodiment of Spanish South America and its unique culture, that's why that proposal is constantly alive. Argentina would start feeling out of place for me, but I'd still prefer it over Australia and Canada as it simply is something more exotic and unique to me (I'd even argue more culturally influential, due to its literature, tango etc).
England, America and Canada
or Australia would be acceptable, or Spain, Mexico and Argentina
or Gran Colombia, just please no four sister civs in one game
And I don't give a damn about perfectionist TSL distribution balance. Earth has been extremely unequal in population density and historical significance, accept that, especially as it doesn't affect gameplay beyond one very specific map type and size.