Gms: how do you keep a pace up?
As most have said, frequent posting helps a lot, as does high-quality posting. The more you make things interesting, the more invested the players. Having a required posting per week helps, but there is a lot of real-life interference that can come up. Being prepared by having SOPs for each character (for default actions) in case of absence lets you carry on the game during someone's short hiatus.
Group size I have found to be a variable factor. I've had large games that folded, I've had small games that have folded. I've had the reasons for that being mostly player attrition from real-life things, a couple of times from inter-player conflict that they refused to resolve, a few times were my fault for lack of posting, and sometimes people just straight-up vanished.
My longest-running game has a cast of 9 and has been running for nearly four and a half years. That game has lasted due to a multitude of factors. I'm running D&D 3.5, so I have a large player pool to pull from. After the first round of inevitable dropouts, I stopped adding or replacing characters and just had new players take over abandoned characters. Then I no longer had to bring the story to a screeching halt to bring in new people. Though I designed the game in an episodic nature (the characters are part of an Adventurer's Guild which take various jobs), the three jobs they've taken so far have ranged from a year to two years in real-time. It would have been very difficult for a new character to show up in the middle of a tomb or a swamp, which was when I decided to stop changing characters and just start replacing players instead. I let players retool the character if they want to (within broad limits), which gives them some customization and agency, but I don't have something jarring like our barbarian suddenly disappearing and being replaced with a rogue.
So, in my view, game longevity can be a combination of good player pool, good story, and both a GM and players' abilities to adapt and compromise (with both posting rates and absences).