The thing is though, FM4 had a Prius in it (as well as a Volt), along with some other basic transport like the Aygo and the Yaris. I don't think it was an issue back then at all. Besides, what Forza is missing as of late are the semi-sporty sedans that were featured in the past (Ford Taurus SHO, Ford Fusion Sport, Chevy Impala SS, etc.) and so something like, whatever the current sporty Camry model is, would fit in.
Clearly back then both T10 and Toyota had different priorities. Namely, in 2011 Toyota was quite content with their image as the world's leading manufacturer of conventional and hybrid econoboxes. Now only showing up with Yarises and Camrys would reflect poorly on their reputation as a sportscar maker. I wouldn't be surprised, however, to see sportier models of their more pedestrian cars find their way back in car games in a more remote future - after they'll manage to show they got their mojo back with whatever will come off the collaboration with BMW and the development of the S-FR. On the other hand, T10 was clearly trying to find a balance in their carlist - with FM5 they cleared their house, and now they're going in a different direction (only including underpowered oddballs when they're vintage and sufficiently iconic, or so it'd seem). If anything, I wish they could differentiate Horizon and Motorsport even further, by adding more pedestrian cars to the former's roster and
maybe trimming the latter's car list a bit.
If I was Toyota, I'd consider the legacy nature of the SW20, AE86 and A70 were good enough reason to cancel the license. They're hardly doing Toyota's intellectual property justice, especially the state the MR2 was in.
This could also be a factor. The SW20 MR2 has basically become a running joke (and the source of much frustration) with the community. However, many other Toyotas in the game (like the A80 Supra, the A20 Celica, the E20 Corolla and the AW10 MR) are/were excellent; if Toyota had an issue with the way some specific models were implemented, it would've made more sense to not license them (and them only) until T10 adressed their many quality issues.
The GT86 is relatively new and it demonstrates they still have it in them. Look at Honda and Nissan compared to then and now. Like Toyota, they are still selling a sportscar or two, but the Silvia, Prelude and S2000 are all living on in Forza. Toyota would be really strange for seeing things differently.
The difference is that Nissan is also selling the 370Z and the GT-R: both cars are the continuation of the heritage established with the vintage cars Forza features. Honda is still making the Civic Type-R and the NSX, Mazda's MX5 is as healthy as ever, and Mitsubishi is too; on the other hand, Toyota don't independently produce any sportscar (the 86 is produced at Subaru's Gunma plant, and is powered by an engine which uses Subaru's trademark boxer layout), and have in fact killed all their "heritage" nameplates. It's a different situation. And you have to consider the fact that they're preparing to against their carefully constructed reputation as a "sensible" car maker by launching two sports models - for which they'll have to build a new legacy, because they've thrown the old one to the weeds.
But of course we're all just speculating, trying to make sense of a baffling decision more than anything. Any guess is as good as any other, especially at this stage. Maybe we'll know more tomorrow, and maybe we'll see Toyota again the next game, or the game after that, or maybe even in DLC, who knows.