Kantor, nothing freaking fits your criteria.
* is a famous series
* is on a decline
* has an earlier entry that will be on many member's lists
* has only one main competitor which is more popular and more realistic.
Only sports or car games come close but not them either. I'd say Need for Speed but that hasn't got a more popular and realistic competitor.
I'd say PES but someone mentioned it already.
It is clearly Tony Hawk. Skate is the one competitor.
Forza or Gran Turismo?
No. Neither are on a decline and both have more than one competitor (NFS)