You really should explain more on why you think your campaign-starts have petered out?
Is it because the players don't have enough free time? Or because they aren't interested in learning new rules? Maybe they don't have time or energy, with life and all?
I'm asking because Savage Worlds, AFAIK, is a rather inviting simple game. Warhammer Fantasy 1E and 2E are too. You wouldn't expect such a game to flounder on basis of rules complexity.
(If you attempted to run WFRP4, on the other hand, you have my sympathies - that game is in my opinion a trainwreck of epic proportions. I even think PF2 is easier to learn and teach, which really is saying something)
Of course, there might be other reasons. Chiefly what DWChancellor says - if you don't like a system as a Games Master, nothing will help. But of course, there might be other issues - such as creating characters that doesn't gel with the story, or the chosen campaign story not engaging the players, etc.
Anyway, you asked about Pathfinder 2. I would say it sets a very high bar for beginners. It is very complex to master, and not a suitable intro game at all.
If you just want to play the game casual, well, you can't. Unless you engage with the myriad (very) small things, I would assume the game loses most of its raison d'etre.
Everything is codified and given specific parameters in this game. As a GM, any time you try to be generous and "just say yes" you will likely have wrecked a feat that gives a character the power you just handed out for free. Basically, your players need to love finding +1 bonuses. I would guess most 5E players would be intensely turned off by the idea their characters should make an effort just to gain a "measly" +1 bonus here or there, but that's considered very valuable and attractive by PF2.
On one hand, PF2 should attract gamers dissatisfied with the relative dearth of build option (crunch) in 5th Edition. On the other, the game unfortunately went in a locked-down direction which does share a certain resemblance to D&D 4. This can be very frustrating for players accustomed to the wonderful freedom of D&D 3 and Pathfinder 1. (A freedom that also led to fantastically broken combos, of course).
There are multiple reasons why campaigns have fizzled over the years, but let's look at the most recent few months so I can focus on more specific issues.
Two players were very interested in going back to 4e D&D for nostalgic reasons. The other players weren't against trying it. So I decided to run it with a focus on roleplaying, story, politics, and investigation to show that 4e isn't all combat. At first, I had the buy-in from all the players and had quite a large group. Then gradually people left. One player left because of his work schedule. Another left for child care reasons. Another just flaked out and stopped coming (even though she promised she still wanted to play and was "very interested in the story.")
By that point, we were down to 3 regular players (and the occasional 4th and 5th sometimes flaked out). Then I shifted the game to Savage Worlds (at the suggestion of "the regular player who likes to switch systems"). Since we didn't have regular attendance, we thought playing a classless system would be better than 4e that had very defined roles - and some sessions we could be missing a defender or striker - or both. We converted the setting and characters into Savage Worlds.
And then I had the issue of not really knowing how to convert to Savage Worlds. We had a couple rough sessions, and maybe people were getting frustrated that the plot wasn't moving quickly enough. One of the regular players said he was leaving to find a "regular 5th edition game."
I begged the players to come to one last session to wrap up the campaign, built a huge 3D castle, spent days on everything. 2 players came. Because of the bad attendance the campaign ended in a TPK. I guess I was happy to be done with it.
I had been running Savage Rifts on another night. Two players "got too busy" to play and stopped coming. Since the core group wasn't around, we decided to try out the Enemy Within campaign for Warhammer 4e with the remaining players. Then, after one session, one of those players had to move to a different city for work. The 3 remaining players decided it wasn't worth continuing.