iMac with 5K Display Update?!
I'm pretty sure most people DO NOT access more than 100GB of material on their internal hard drive
As others have said, that's just simply not true. Someone with 700GB of data, such as yourself, will likely use more than 100GB on a fairly regular basis.
If you don't - you're proving my point - why keep it on a desktop computer? Store it on a network drive, share it across all your devices, all the time. The speed difference between a network attached SSD and your 7200rpm Fusion drive will be negligible, if not favoured towards the NAS. And the NAS speeds are only going to get better.
It doesn't even come down to affordability. The 256GB SSD is the same price as a Fusion drive. Put all your media on a NAS, and keep ~200GB of apps, frequent files, and ongoing work projects on the iMac. The external storage could even be a 7200rpm drive - it doesn't matter, the bottleneck will be moved outside of your $3,000 iMac and can be alleviated at ease later, without needing to drop $3k.
So even if you have 2TB of data, a 256GB SSD is still the better option, because you're getting ~220GB of your data on super fast SSD, vs. 100GB. You can later extend that to xTB of thunderbolt/SSD storage, which is incomparably faster than the Fusion HDD. With the Fusion drive, you're forever stuck with a bottleneck that predates the 2000s, or indeed an empty spinning hard drive inside your expensive iMac.
As far as I see it, a Fusion drive doesn't make sense financially or practically, which is why I was recommending an SSD like many others. That's all.
It's not subjective stuff I'm putting down here. I myself am on a DYI fusion drive with a 240GB SSD and 1TB HDD, and so I understand the shortfalls. Since I got my NAS, my internal HDD is pretty much used as a temporary drive.