Actually, the main purpose is for app development for iOS. Not sure that requires a lot of firepower. Most of my home music listening is on this, my Windows machine, and most of my work and travel music listening is on mobile devices, FLAC and Tidal cached streaming. So back to the Mac Mini -- no need for super high power GPU assisted computing, or video power, or audio power. It'll be largely for an IDE and not much more. If I wanted that, I'd probably get the new iMac Pro, but I don't have that kind of money. I kind of thought software had a longer lifespan on Macs, so I didn't think 2014 was that long ago, "in dog years", as it were.
But yes, I think I'd be a little too frustrated with the 1.4.
And yes, that's what I was thinking -- I wouldn't need as much cache memory if I had a fusion drive...I think. I've actually not ever designed a system around compromises.