General principal of economy changes over the years?

[BoredEngineer]

I've started in 1750 and actively developed and tuned network for about 14 years. Then decided to skip some time and wait for some historical revolutionary unlocks, just running game full speed in the background.
I was expecting profitability to go a bit down as I'm not actively managing network but what I see now surprises me:
https://www.dropbox.com/s/dv0f6b4cdn1uekv/Yearly_Changes.JPG
While transportation didn't change much, my operational profit and revenue went down. On closer inspection, some of the passenger lines which where very busy and typically had nearly full stops, now are very much empty.
The "transportation" might be slightly misleading as I had huge amount of mail being transported, perhaps mail just masks the drop in passenger transportation.
95% of transportation is passenger and mail coaches, the rest are ships and some individual cases of dairy/vegetable delivery.
Sadly I don't see how statistics can be viewed per line on yearly basis, perhaps the issue is just few big lines and not some general trend.

As far as roads come, I've got only two improvements, a cobblestone roads, which provide a maximum speed of 35km/h that I have no means to utilize and a slightly more comfortable coach.
Do people and mail is just payed less as time progresses? Do more people start to use private transport? Do they care that some vehicles are 20 years old?

Save game just in case:
https://www.dropbox.com/s/6rfsa7ryry08rbb/Campaign%202.sve

[PJMack]

There is an increase in private "car" use, but during the 1760s, the increase is, according to the config file, nearly negligible.  There is also a real-time processing delay between the time a new road is build and the private cars are aware of it, however for your map (with an i7-10700K), that delay is only a few seconds.  (On very large maps in the modern era, it can take 24 real life hours to re-calculate private car routes).  I also checked, and no new factories created any shortcuts for private cars.

For every new line, there is an initial surge in passengers before usage stabilizes.  This usually only takes a few cycles of vehicles traversing the route, not five in-game years for road vehicles.  One possibility is that many of the passengers/mail could have been going to/from the docks as a side effect to the initial rush to shipping line which can take a few years to stabilize.  Even when shipping is "stable" there does tend to be cycles of revenue to non-revenue due to the nature of bulk transport via a slow medium.

As for going forward, you may want to begin adding more lines to new cities, as well as interspersing new vehicles into existing lines to increase service frequency.