I feel as though this sort of fragmentation hurts certain other games and genres, with first-person shooters coming immediately to mind. Unsurprisingly, most times I played Forza Horizon games with anyone online, those playing with me didn't own the expansions. This got me to thinking about the popularity (or lack thereof) of the expansions throughout the series, so I decided to look at achievement completion rates for the first achievement for each game's expansion, and I've detailed them below.
I assume this to mean that only 5.41% of people who own the Forza Horizon 2 base game even played the expansion, as I can't imagine that so many people would own the expansion and never bother playing through the intro. Earlier this week, I decided to start Forza Horizon 2 from the beginning again, and I finally gave Storm Island a shot and was surprised when the very first achievement for reaching the hub popped, as it registered as a rare achievement with a whopping 5.41% completion. I admittedly never got too "into" the expansions until Forza Horizon 4's Fortune Island expansion, and it has some of my favorite routes in the series. Recently, I pre-ordered Forza Horizon 5 Premium Edition. At some point, I even played through both Forza Horizon 2 and 3. I started with Forza Horizon 3, and it made a strong enough impression on me that I ended up pre-ordering Forza Horizon 4 Ultimate Edition.