Updated Eternal Massilia with test free run for over 1800 years. Took ca. 9.5 hours of real-life time on 100% game speed. There are number of consecutive saves for the lovers of precise analysis:
1) 270 AD and 309 AD - the start of free run, during which some minor interventions and small city changes were made.
2) 1500-1998 - just some consecutive random saves of free run (with notable dates like 1914 (WWI), 1998 (first C3 release) :)).
3) Jan 2021 - current time which is the last save.
The city shall not generate more that 52 ghosts (already 19 ghosts formed) in 100 000 years. 92 culture shall be stable for eternity as there are 37 unemployed workers (33 ghosts to be additionally formed are less than 37 free workers). Census curve has been fully stabilized around 500 AD and does not change a lot during 1500 years. There were only minor fluctuations connected with ghosts arising.
I did not ever find in downloads section the eternal Massilia with 92 culture which passed 2000 AD date in free run mode, so I consider this city as truly eternal. Pretty sure that if during 1800 year cycle nothing happened, then it will not happen for any other period no matter how long it is. There shall not be any undiscovered super-deep long cycles in walkers behavior or some unique combination of events which may cause city collapse. If they are the case, they would already arise during 1800 years (and ca. 1500 years of completely stable census).